fix(component): normalize path separators

[rami3l]

Supersedes #3077.

This solution is based on the analysis made in #3077 (comment).

PS: Sincerely, I think this ComponentPart needs to be changed to something like:

struct ComponentPart {
  pub kind: String,
  pub path: PathBuf,
}

... I'd like to work on stylistic changes like this once I'm done with updating the test cases (some of them are of course expected to break).
Done.

[ChrisDenton]

Thanks! I think this will be more consistent between platforms.