Fix harness getDirectores implementation to not include directory as prefix

[ghost]

Fixes #21100 (comment)

We already had fourslash tests for getting directories in path completions. But, the harness implementation of getDirectories differed from the original one, as it returned directories with the prefix present.
Now directory completions with paths should for real in addition to in fourslash tests. (See the linked comment for repro instructions.)

[rbuckton]

I am completely replacing the implementation of getDirectories in #20763, so we'll need to coordinate this change.

[ghost]

I went with standardizing returning "foo" instead of "/directory/path/foo" because all getDirectories implementations but the one in harnessLanguageService seemed to be doing that already. So long as your PR also keeps that behavior in all getDirectories implementations I don't think there will be any conflict.

[rbuckton]

I'll update my PR after this is in.