fix(@angular-devkit/build-angular): handle windows spec collisions

[jkrems]

The previous logic didn't quite work when interacting with windows path. This PR does three things:

1. Move the logic to generate unique spec entrypoint names into its own function. This prepares for reusing it elsewhere (e.g. in WTR).
2. Add a more direct unit test for that logic.
3. Fix the logic so it works with Windows paths as well as unix paths.

As a drive-by fix, this also cleans up the deduped filenames so we get "foo-2.spec.js" instead of "foo.spec-2.js". The latter looked a bit confusing and it was just a tiiiiny regex to fix those names up. The index append shouldn't really come up in real code very often, so this is very much a tiny niche case.

Fixes #28915

[alan-agius4]

The changes were merged into the following branches: main, 19.0.x

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