Move RepoHook class from project.py file to dedicated file

The project.py file is huge and contains multiple
classes. By moving it to seperate class files the code
becomes more readable and maintainable.

Signed-off-by: Remy Bohmer <github@bohmer.net>
Change-Id: Ida9d99d31751d627ae1ea0373418080696d2e14b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/281293
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Remy Bohmer <linux@bohmer.net>