Deprecate MemoryResourceProvider.convertPath in favor of extension

Work towards https://github.com/dart-lang/sdk/issues/55660

The comment for `MemoryResourceProvider.convertPath` includes:

> This is a utility method for testing; paths passed in to other
> methods in this class are never converted automatically.

and indeed, the actual impl of this method is found in an extension,
in analyzer's test_utilities/ directory. It seems to me better to
leave a testing utility as a testing utility, and not expose it in
the public API of MemoryResourceProvider.

Additionally, the extension method is used by `ResourceProviderMixin`,
which is moving to the public analyzer_testing package. It is illegal
to have a circular non-dev dependency between the analyzer package
and the analyzer_testing package.

The migration for the ~half dozen test files that use this method is
to call the extension method directly. Sometimes with an extension
override, and sometimes without.

Change-Id: I9c2e18600461134bfd91c082b3af0d5079600f0c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/426984
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>