BUILD file symbols help info (pantsbuild#18378)

Adds `symbols` as a new help topic for BUILD file symbols documentation.

Supports both builtin symbols as well as macros defined in prelude files.

<img width="773" alt="image"
src="https://user-images.githubusercontent.com/72965/222631240-95285468-28b2-4b31-a91a-32bd48884beb.png">

src/python/pants/build_graph/build_configuration.py

@@ -30,7 +30,16 @@
 
 # No goal or target_type can have a name from this set, so that `./pants help <name>`
 # is unambiguous.
-_RESERVED_NAMES = {"api-types", "backends", "global", "goals", "subsystems", "targets", "tools"}
+_RESERVED_NAMES = {
+    "api-types",
+    "backends",
+    "global",
+    "goals",
+    "subsystems",
+    "symbols",
+    "targets",
+    "tools",
+}
 
 
 # Subsystems used outside of any rule.

src/python/pants/engine/internals/build_files.py

@@ -31,14 +31,19 @@
     MaybeBuildFileDependencyRulesImplementation,
 )
 from pants.engine.internals.mapper import AddressFamily, AddressMap
-from pants.engine.internals.parser import BuildFilePreludeSymbols, Parser, error_on_imports
+from pants.engine.internals.parser import (
+    BuildFilePreludeSymbols,
+    BuildFileSymbolsInfo,
+    Parser,
+    error_on_imports,
+)
 from pants.engine.internals.session import SessionValues
 from pants.engine.internals.synthetic_targets import (
     SyntheticAddressMaps,
     SyntheticAddressMapsRequest,
 )
 from pants.engine.internals.target_adaptor import TargetAdaptor, TargetAdaptorRequest
-from pants.engine.rules import Get, MultiGet, collect_rules, rule
+from pants.engine.rules import Get, MultiGet, QueryRule, collect_rules, rule
 from pants.engine.target import (
     DependenciesRuleApplication,
     DependenciesRuleApplicationRequest,
@@ -89,7 +94,8 @@ async def evaluate_preludes(
     globals: dict[str, Any] = {
         **{name: getattr(builtins, name) for name in dir(builtins) if name.endswith("Error")},
         # Ensure the globals for each prelude includes the builtin symbols (E.g. `python_sources`)
-        **parser.builtin_symbols,
+        # and any build file aliases (e.g. from plugins)
+        **parser.symbols,
     }
     locals: dict[str, Any] = {}
     env_vars: set[str] = set()
@@ -112,6 +118,15 @@ async def evaluate_preludes(
     return BuildFilePreludeSymbols.create(locals, env_vars)
 
 
+@rule
+async def get_all_build_file_symbols_info(
+    parser: Parser, prelude_symbols: BuildFilePreludeSymbols
+) -> BuildFileSymbolsInfo:
+    return BuildFileSymbolsInfo.from_info(
+        parser.symbols_info.info.values(), prelude_symbols.info.values()
+    )
+
+
 @rule
 async def maybe_resolve_address(address_input: AddressInput) -> MaybeAddress:
     # Determine the type of the path_component of the input.
@@ -480,4 +495,10 @@ async def get_dependencies_rule_application(
 
 
 def rules():
-    return collect_rules()
+    return (
+        *collect_rules(),
+        # The `BuildFileSymbolsInfo` is consumed by the `HelpInfoExtracter` and uses the scheduler
+        # session `product_request()` directly so we need an explicit QueryRule to provide this type
+        # as an valid entrypoint into the rule graph.
+        QueryRule(BuildFileSymbolsInfo, ()),
+    )