Port the plugin resolver test to v2 test style. (#10424)

Previously it used some hackery to get hold of various
subsystems to construct a pex to run setup.py, in order
to create the plugin dists it then tested against.

This change uses v2 test idioms to create and run
that pex.

As a result, we can delete a lot of code that only
hung around because of this one test.

Unfortunately, this required making the test functions
methods of a test class, meaning we can no longer
use pytest.mark.parameterize, and have to "parameterize"
manually.

[ci skip-rust-tests]

src/python/pants/backend/python/rules/inject_ancestor_files.py

@@ -2,14 +2,15 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 import itertools
+import os
 from dataclasses import dataclass
 from pathlib import PurePath
+from typing import Sequence, Set
 
 from pants.core.util_rules.strip_source_roots import SourceRootStrippedSources, StripSnapshotRequest
 from pants.engine.fs import EMPTY_SNAPSHOT, PathGlobs, Snapshot
 from pants.engine.rules import rule
 from pants.engine.selectors import Get
-from pants.python.pex_build_util import identify_missing_ancestor_files
 from pants.source.source_root import AllSourceRoots
 from pants.util.ordered_set import FrozenOrderedSet
 
@@ -41,6 +42,32 @@ class AncestorFiles:
     snapshot: Snapshot
 
 
+def identify_missing_ancestor_files(name: str, sources: Sequence[str]) -> FrozenOrderedSet[str]:
+    """Return the paths of potentially missing ancestor files.
+
+    NB: If the sources have not had their source roots (e.g., 'src/python') stripped, this
+    function will consider superfluous files at and above the source roots, (e.g.,
+    src/python/<name>, src/<name>). It is the caller's responsibility to filter these
+    out if necessary. If the sources have had their source roots stripped, then this function
+    will only identify consider files in actual packages.
+    """
+    packages: Set[str] = set()
+    for source in sources:
+        if not source.endswith(".py"):
+            continue
+        pkg_dir = os.path.dirname(source)
+        if not pkg_dir or pkg_dir in packages:
+            continue
+        package = ""
+        for component in pkg_dir.split(os.sep):
+            package = os.path.join(package, component)
+            packages.add(package)
+
+    return FrozenOrderedSet(
+        sorted({os.path.join(package, name) for package in packages} - set(sources))
+    )
+
+
 @rule
 async def find_missing_ancestor_files(
     request: AncestorFilesRequest, all_source_roots: AllSourceRoots

src/python/pants/backend/python/rules/inject_ancestor_files_test.py

@@ -7,6 +7,7 @@
     AncestorFiles,
     AncestorFilesRequest,
     find_missing_ancestor_files,
+    identify_missing_ancestor_files,
 )
 from pants.core.util_rules import strip_source_roots
 from pants.engine.fs import DigestContents
@@ -89,3 +90,29 @@ def test_stripped(self) -> None:
             ],
             expected_discovered=["project/__init__.py"],
         )
+
+
+def test_identify_missing_ancestor_files() -> None:
+    assert {"a/__init__.py", "a/b/__init__.py", "a/b/c/d/__init__.py"} == set(
+        identify_missing_ancestor_files(
+            "__init__.py", ["a/b/foo.py", "a/b/c/__init__.py", "a/b/c/d/bar.py", "a/e/__init__.py"]
+        )
+    )
+
+    assert {
+        "src/__init__.py",
+        "src/python/__init__.py",
+        "src/python/a/__init__.py",
+        "src/python/a/b/__init__.py",
+        "src/python/a/b/c/d/__init__.py",
+    } == set(
+        identify_missing_ancestor_files(
+            "__init__.py",
+            [
+                "src/python/a/b/foo.py",
+                "src/python/a/b/c/__init__.py",
+                "src/python/a/b/c/d/bar.py",
+                "src/python/a/e/__init__.py",
+            ],
+        )
+    )