[skip] fix match class + path matching in reporting of unused skips

[TomasVotruba]

Bug

UnusedSkipResolver::resolveUnusedRuleScopedSkips() matched used skips by path only, ignoring the rule class.

Used skips live in one flat list (UsedSkipCollector), mixing rule-scoped paths and global paths. The same path skipped under two rules collapsed to a single key, so a match under one rule silently marked the same path under every other rule as "used".

Before (false negative)

->withSkip([
    SomeRector::class    => ['src/Foo.php'], // matched a file -> used
    AnotherRector::class => ['src/Foo.php'], // never matched -> still UNUSED
])

src/Foo.php matched under SomeRector, landed in the used list as the bare path src/Foo.php. The resolver then checked in_array('src/Foo.php', $usedSkips) for AnotherRector too — found it — and reported the genuinely unused skip as used. It was never flagged for removal.

After (fixed)

Used skips are now tracked scoped to their rule as class|path:

• SkipSkipper::doesMatchSkip() marks $skippedClass . '|' . $matchedPath
• UnusedSkipResolver checks in_array($rectorClass . '|' . $path, $usedSkips, true)

SomeRector|src/Foo.php and AnotherRector|src/Foo.php are distinct keys, so the unmatched skip under AnotherRector is correctly reported as unused.

Global skips and skip-everywhere (null path) rules are unchanged — they carry no rule scope.

Tests

• New regression testSamePathUnderAnotherRuleDoesNotMarkSkipUsed — same path under two rules, only one matched; asserts the other still reports unused.
• Updated UnusedSkipResolverTest + UsedSkipCollectorTest to the class|path key shape.