fetchRepoProject: Fetch into $out and make it deterministic

Fetch into $out and remove all version control files to make it
deterministic (.repo and all .git subdirectories - e.g. the .git/index
files change every time).

Additionally I've changed the default of "useArchive" to false because
fetching with "--archive" will fail for some projects (e.g.
"platform/external/iosched" from the AOSP).

Now, this function should hopefully work for every tag of the AOSP.