source-repositories does a git reset on the .git directory of cached
repositories. But doesn't specify the directory to reset. A working
directory needs to be specified so that the $PWD isn't used.
Without this change $PWD is polluted with the contents of repositories
being cached.
Change-Id: Ic37b702ac579bf766bb2204a988fa9468d308abf
Caching PyPI objects will require inspecting the content downloaded by
the source repositories, so needs to run after it.
Change-Id: I551c075e0bb6853b2ef79268b7f6455a4f06743d