internal: add an interface for internal/postgres.DB

And use that interface to remove the dependency from internal/frontend
to internal/postgres.

The dependency still exists in the test code. That will be taken care
of in another CL.

For #61399
Change-Id: Ic1b3c257e6eb34858cf31b7596251aaff71d3482
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/510417
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>

0 files changed, 0 insertions, 0 deletions