Mark editable-installed plugins for assertion rewriting

[GangEunzzang]

Problem

_mark_plugins_for_rewrite() decides which modules to mark for assertion rewriting by walking the Python files recorded in each plugin distribution's metadata. A PEP 660 editable install doesn't record them — dist.files only holds the __editable__*.pth shim and the dist-info entries — so nothing matches and the plugin never gets marked.

The effect is that assertions inside a plugin stop being rewritten as soon as it is installed with pip install -e ., which is the usual way to work on one. With a plugin that asserts in its own helpers:

# myplug/helper.py
def compare(got, want):
    assert got == want

# pip install .
E       AssertionError: assert {'a': 1, 'b': 2} == {'a': 1, 'b': 3}

# pip install -e .
E       AssertionError

Fix

When a distribution is installed in editable mode and its recorded files yield no rewritable module, fall back to the top-level package of its pytest11 entry points. Editable mode is read from direct_url.json (PEP 610), so regular installs keep taking the existing file-based path untouched.

The direct_url.json gate matters. Without it the fallback also fires for distributions that simply record no files, which a number of tests in the suite construct, and 13 of them change behaviour.

Tests

The new test covers three distributions: an editable one, a regular install from a local directory, and one installed from an index with no direct_url.json. Only the editable case fails on main.

Fixes #11783

[RonnyPfannschmidt]

potential edge-case - this might mark all of a set of namespace packages intead of just the one with the plugin

i'like to see this validated, but i recon its pretty tricky to create the conditions

[GangEunzzang]

@RonnyPfannschmidt Thanks for the review — you were right.

I set the conditions up with two distributions sharing a PEP 420 namespace: nsplug installed editable with a myns.plug.plugin entry point, and an unrelated nsother installed normally. myns did get marked, so an assertion inside myns/other/helper.py came out rewritten:

# main
E       AssertionError

# this branch, before the fix
E       AssertionError: assert {'a': 1} == {'a': 2}

The fallback now walks the entry point module and takes the outermost package that isn't a namespace package, so it stops at myns.plug and leaves myns.other alone. It resolves through PathFinder rather than importlib.util.find_spec, since that one imports the parent package, which would defeat the point of marking it.

Added a unit test covering both shapes.