Enforce backend isolation from core pants--make exceptions explicit. (p…

…antsbuild#18093)

Cut of the pants backends from the rest of the code base, with
exceptions for already existing bleed.

Per the very good idea by @benjyw on the last team meetup.

src/python/pants/backend/BUILD

@@ -2,3 +2,116 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 python_sources()
+
+
+# Setup rules about which parts of the codebase may depend on backend sources.
+
+DEFAULT_DEPENDENTS_RULES = (
+    # Allow dependencies within/between backends.
+    "/**",
+    # Allow the pants bin plugins target to depend on backends.
+    "src/python/pants/bin:plugins",
+    # Allow test utils and fixtures to depend on code in backend modules.
+    "/../testutil/**",
+    "//pants-plugins/internal_plugins/test_lockfile_fixtures/*",
+    # Allow test sources to depend on code in backend modules.
+    "<*_tests>",
+    "//tests/**",
+    # Allow build-support to use what ever.
+    "//build-support/**",
+    # Deny all others.
+    "!*",
+)
+
+
+__dependents_rules__(
+    # Exceptional rule sets:
+    (
+        (
+            "[/build_files/fix/deprecations/base.py]",
+            "[/build_files/fix/deprecations/renamed_fields_rules.py]",
+            "[/build_files/fix/deprecations/renamed_targets_rules.py]",
+            "[/build_files/fmt/black/register.py]",
+            "[/build_files/fmt/yapf/register.py]",
+            "[/python/lint/black/rules.py]",
+            "[/python/lint/black/subsystem.py]",
+            "[/python/lint/yapf/rules.py]",
+            "[/python/lint/yapf/subsystem.py]",
+        ),
+        "src/python/pants/core/goals/update_build_files.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        (
+            "[/python/util_rules/interpreter_constraints.py]",
+            "[/python/util_rules/pex_environment.py]",
+            "[/python/util_rules/pex_requirements.py]",
+        ),
+        "src/python/pants/init/plugin_resolver.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/python/util_rules/pex.py]",
+        "src/python/pants/core/goals/update_build_files.py",
+        "src/python/pants/init/plugin_resolver.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/python/goals/setup_py.py]",
+        "//pants-plugins/internal_plugins/releases/register.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/project_info/filter_targets.py]",
+        "src/python/pants/build_graph/build_configuration.py",
+        "src/python/pants/engine/internals/specs_rules.py",
+        "src/python/pants/engine/internals/mapper.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/project_info/dependents.py]",
+        "src/python/pants/vcs/changed.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/project_info/peek.py]",
+        "src/python/pants/explorer/server/graphql/**",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/project_info]",
+        "src/python/pants/init/extension_loader.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/java/subsystems/java_infer.py]",
+        "src/python/pants/jvm/dependency_inference/artifact_mapper.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/java/subsystems/junit.py]",
+        "src/python/pants/jvm/test/junit.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/docker/target_types.py]",
+        "src/python/pants/explorer/server/graphql/query/conftest.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/codegen/export_codegen_goal.py]",
+        "src/python/pants/core/register.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    (
+        "[/__init__.py]",
+        "src/python/pants/help/help_info_extracter.py",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+    # Generic rule set:
+    (
+        # Applies for all targets in this subtree.
+        "*",
+        DEFAULT_DEPENDENTS_RULES,
+    ),
+)