Fix package ID collisions for remapped inputs

[jakebailey]

Fixes #4758

See the second commit for the diff; note how the set reference mapped to get before but now correctly maps to set.

[Copilot]

Pull request overview

Fixes package-ID collisions when declaration outputs remap to source files outside the package directory.

Changes:

• Omits invalid package IDs for remapped external inputs.
• Correctly handles trailing package-directory separators.
• Adds regression tests and baselines for colliding and non-colliding paths.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/module/resolver.go	Validates resolved paths before constructing package IDs.
testdata/tests/cases/compiler/packageSelfNameSubpathExportsPackageIdCollision.ts	Tests distinct package IDs at the filesystem root.
testdata/tests/cases/compiler/packageSelfNameSubpathExportsPackageIdOutsidePackage.ts	Tests remapped inputs outside the package.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdCollision.types	Captures inferred types.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdCollision.trace.json	Captures distinct package IDs.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdCollision.symbols	Captures resolved symbols.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdCollision.js	Captures emitted output.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdCollision.errors.txt	Captures expected diagnostics.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdOutsidePackage.types	Captures corrected inferred types.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdOutsidePackage.trace.json	Confirms remapped paths omit package IDs.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdOutsidePackage.symbols	Captures corrected symbols.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdOutsidePackage.js	Captures emitted output.
testdata/baselines/reference/compiler/packageSelfNameSubpathExportsPackageIdOutsidePackage.errors.txt	Confirms set resolves independently.

[jakebailey]

I might have been bamboozled, I think this bug only happens if packageDirectory is / which the test case can make happen, but is very likely not reality for the real repro.