internal/fetch: add two stdlib getters

This breaks out the behavior of getting the stdlib into its own
getter, stdlibZipModuleGetter. Most (but not all) of the special cases
in the fetch code for the stdlib are in the stdlibZipModuleGetter,
which is installed on the frontend. For the local pkgsite command, we
add a new builder for the goPackagesModuleGetter that loads the
packages from the stdlib given a GOROOT. The stdlibZipModuleGetter is
also installed for pkgsite in as a fallback if the
goPackagesModuleGetter doesn't successfully fetch the stdlib module
(I'm not sure if that's possible).

Fixes #60114

Change-Id: Ida6a5367343643cc337c6d05e0d40095f79ca9e5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/507883
Reviewed-by: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
kokoro-CI: kokoro <noreply+kokoro@google.com>

0 files changed, 0 insertions, 0 deletions