Use posix path style for relative imports

[Sese-Schneider]

Details

Fixes #711

Generated relative imports paths on windows used backslashes instead of forwards slashes.
This broke the DartFormatter and the build_runner aborted.

Fix

I specifically added p.Context(style: p.Style.posix) to the .relative part of the path generation.
This ensures the import paths contain only forward slashes.

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[Sese-Schneider]

Interesting sidenote:
I don't know why this broke for me suddenly. We've been using mockito for over 2 years now and everything used to work fine.
Furthermore we haven't updated the dart version in our codebase in the last few months. The issue seem to have appeared out of nowhere 🤷🏼‍♂️