Add back ignore for --files-not-found-behavior (pantsbuild#9175)

### Problem

1.25.x already made a big breaking change by deprecating `globs()`. When paired with this, it proved too difficult for some users to upgrade. We will eventually deprecate `--ignore`, but only after `globs()` is removed.

### Solution

- Add back the `'ignore'` case to `--files-not-found-behavior`, which is converted into a `GlobMatchErrorBehavior` before being injected into the v2 rule graph as a singleton.

src/python/pants/base/deprecated.py

@@ -195,6 +195,7 @@ def deprecated_conditional(
     removal_version: str,
     entity_description: str,
     hint_message: Optional[str] = None,
+    deprecation_start_version: Optional[str] = None,
     stacklevel: int = 4,
 ) -> None:
     """Marks a certain configuration as deprecated.
@@ -211,7 +212,13 @@ def deprecated_conditional(
     """
     validate_deprecation_semver(removal_version, "removal version")
     if predicate():
-        warn_or_error(removal_version, entity_description, hint_message, stacklevel=stacklevel)
+        warn_or_error(
+            removal_version,
+            entity_description,
+            hint_message,
+            deprecation_start_version=deprecation_start_version,
+            stacklevel=stacklevel,
+        )
 
 
 def deprecated(

src/python/pants/engine/fs.py

@@ -55,8 +55,8 @@ class PathGlobs:
 
     The syntax supported is roughly Git's glob syntax.
 
-    NB: this object is interpreted from within Snapshot::lift_path_globs() -- that method will need to
-    be aware of any changes to this object's definition.
+    NB: this object is interpreted from within Snapshot::lift_path_globs() -- that method will need
+    to be aware of any changes to this object's definition.
     """
 
     globs: Tuple[str, ...]
@@ -76,9 +76,9 @@ def __init__(
                       with `!`, e.g. `!ignore.py`.
         :param glob_match_error_behavior: whether to warn or error upon match failures
         :param conjunction: whether all `include`s must match or only at least one must match
-        :param description_of_origin: a human-friendly description of where this PathGlobs request is
-                                      coming from, used to improve the error message for unmatched
-                                      globs. For example, this might be
+        :param description_of_origin: a human-friendly description of where this PathGlobs request
+                                      is coming from, used to improve the error message for
+                                      unmatched globs. For example, this might be the text string
                                       "the option `--isort-config`".
         """
         self.globs = tuple(globs)

src/python/pants/engine/legacy/graph.py

@@ -717,7 +717,11 @@ async def hydrate_sources(
         glob_match_error_behavior=glob_match_error_behavior,
         # TODO(#9012): add line number referring to the sources field. When doing this, we'll likely
         # need to `await Get[BuildFileAddress](Address)`.
-        description_of_origin=f"{address}'s `{sources_field.arg}` field",
+        description_of_origin=(
+            f"{address}'s `{sources_field.arg}` field"
+            if glob_match_error_behavior != GlobMatchErrorBehavior.ignore
+            else None
+        ),
     )
     snapshot = await Get[Snapshot](PathGlobs, path_globs)
     fileset_with_spec = _eager_fileset_with_spec(
@@ -740,7 +744,11 @@ async def hydrate_bundles(
             glob_match_error_behavior=glob_match_error_behavior,
             # TODO(#9012): add line number referring to the bundles field. When doing this, we'll likely
             # need to `await Get[BuildFileAddress](Address)`.
-            description_of_origin=f"{address}'s `bundles` field",
+            description_of_origin=(
+                f"{address}'s `bundles` field"
+                if glob_match_error_behavior != GlobMatchErrorBehavior.ignore
+                else None
+            ),
         )
         for pg in bundles_field.path_globs_list
     ]

src/python/pants/option/global_options.py

@@ -15,6 +15,7 @@
     get_pants_configdir,
     pants_version,
 )
+from pants.base.deprecated import deprecated_conditional
 from pants.option.custom_types import dir_option, file_option
 from pants.option.errors import OptionsError
 from pants.option.option_value_container import OptionValueContainer
@@ -46,10 +47,23 @@ class GlobMatchErrorBehavior(Enum):
 class FileNotFoundBehavior(Enum):
     """What to do when globs do not match in BUILD files."""
 
+    ignore = "ignore"
     warn = "warn"
     error = "error"
 
     def to_glob_match_error_behavior(self) -> GlobMatchErrorBehavior:
+        deprecated_conditional(
+            lambda: self == type(self).ignore,
+            removal_version="1.29.0.dev2",
+            entity_description="--files-not-found-behavior-option=ignore",
+            hint_message=(
+                "If you currently set `--files-not-found-behavior=ignore`, you will "
+                "need to instead either set `--files-not-found-behavior=warn` (the "
+                "default) or `--files-not-found-behavior=error`. Ignoring when files are "
+                "not found often results in subtle bugs, so we are removing the option."
+            ),
+            deprecation_start_version="1.27.0.dev0",
+        )
         return GlobMatchErrorBehavior(self.value)
 
 
@@ -491,11 +505,7 @@ def register_bootstrap_options(cls, register):
             default=GlobMatchErrorBehavior.warn,
             removal_version="1.27.0.dev0",
             removal_hint="If you currently set `--glob-expansion-failure=error`, instead set "
-            "`--files-not-found-behavior=error`.\n\n"
-            "If you currently set `--glob-expansion-failure=ignore`, you will "
-            "need to instead either set `--files-not-found-behavior=warn` (the "
-            "default) or `--files-not-found-behavior=error`. Ignoring when files are "
-            "not found often results in subtle bugs, so we are removing the option.",
+            "`--files-not-found-behavior=error`.",
             help="What to do when files and globs specified in BUILD files, such as in the "
             "`sources` field, cannot be found. This happens when the files do not exist on "
             "your machine or when they are ignored by the `--pants-ignore` option.",