fix: Saving the root route will correctly update types

[elliott-with-the-longest-name-on-github]

path.dirname was resolving to '.' for the root route, meaning it wouldn't match the '' identifier in the manifest. Easy fix.

Closes #6444.

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[elliott-with-the-longest-name-on-github]

@Rich-Harris

Hang on -- tests may pass, but I didn't add a test to verify this behavior (couldn't figure out how). Do we want to add a test? If no, I need to go manually check that the behavior is working as expected.

[elliott-with-the-longest-name-on-github]

Can confirm on local repro that the changes we just made do work.