Fix kwarg detection for chained receivers (issue #327)

[apoorvdarshan]

Fixes #327

Root cause

For a chained receiver such as Path.cwd().exists(follow_symlinks=True), the resolved
function name includes the intermediate cwd() call segment, so the kwarg is looked up
under ('pathlib.Path.cwd.exists', 'follow_symlinks'). That tuple does not match the rule
key ('pathlib.Path.exists', 'follow_symlinks') in KWARGS_REQS, so the follow_symlinks
kwarg is silently dropped and the minimum version is under-reported.

Reproduction

from pathlib import Path
print(Path.cwd().exists(follow_symlinks=True))

Before (wrong):

$ vermin repro.py
Minimum required versions: 3.4

After (correct):

$ vermin repro.py
Minimum required versions: 3.12

The direct form Path('.').exists(follow_symlinks=True) was already reported correctly as
3.12; only the chained-receiver form was affected. The same problem applied to the other
pathlib methods that gained follow_symlinks in 3.13 (e.g. is_file, is_dir), as noted
in the issue thread.

Fix

In SourceVisitor.visit_keyword, when the fully resolved function name does not match a
known kwarg rule and it contains more than two segments (i.e. a chained call), retry with
the intermediate call segment(s) collapsed away — keeping the receiver plus the final
method (pathlib.Path.exists). Because an already-resolved receiver can span several
segments (e.g. pathlib.Path.cwd.exists), each receiver-length prefix is tried joined with
the final method. A collapsed candidate is only accepted when (name, kwarg) is an actual
entry in kwargs_reqs_rules, so unrelated chains (e.g. foo.bar().exists(...)) cannot
produce false matches. The change is localized to the existing kwarg-resolution loop and
does not alter any of the primary lookup paths.

Tests

Added focused regression tests in tests/kwargs.py for the chained-receiver form of
Path.exists, Path.is_file, and Path.is_dir. They fail on the unmodified code and pass
with the fix. The full existing suite (python3 runtests.py, 4382 tests across 26 suites)
passes, and flake8 is clean on the changed files.

Disclosure: prepared with AI assistance; reviewed and verified locally.