Historically, the build.golang.org was the entire build system, and it
maintained a parallel copy of that git history in its datastore. It
was always buggy and incomplete and things like force pushes were
scary because the datastore mirror could get out of sync. It was also
a lot of code to support that sync.

This changes build.golang.org to instead get the git history from
maintnerd, and then we can remove all the HTTP handlers around
updating it, and can remove all the gitmirror code to call it to
maintain it.

Now build.golang.org only keeps build results, keyed on the commit
hash. It's much less code, but is still an App Engine app for now.
(but it's getting small enough, that porting it to
cloud.google.com/go/datastore is looking very simple)

This also adds a new "repos" package to unify the configuration of the
various Go repos. There were incomplete & redundant copies all over
the place.

Updates golang/go#34744
Fixes golang/go#35828
Fixes golang/go#31236 (New branch=mixed support adds this when desired)
Fixes golang/go#35944

Change-Id: Ifb39417287df3dea052ba8510566d80b4bc75d51
Reviewed-on: https://go-review.googlesource.com/c/build/+/208697
Reviewed-by: Bryan C. Mills <bcmills@google.com>