Strip the enclosing package's lower-case name from a library's dir name

[srawlins]

Fixes #3882

I wish I could write a unit test for this fix but it would be a lot of work, and I fear making too small a unit test (like literally just mocking out a package name to be 'Flutter' and basically just testing toLowerCase) would be unhelpful. To make a less mock-y test, with a real SDK-provided 'Flutter' package, and documenting dependencies, and running dartdoc how Flutter does, would be fragile and very complex.

In any case, I did manually check this fix:

1. Run dart tool/task.dart doc flutter; see that the generated HTML has the flutter/widgets/widgets-library.html path for the widgets library.
2. Edit the flutter checkout that the above command created, in a temp space, to have an unnamed library.
3. Re-run the create_api_docs.dart command that is printed out in the first step; see that the generated HTML still has the correct path.

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[srawlins]

Ready for review, @dart-lang/analyzer-team