go,cmd,internal: update to anticipate missing targets and .a files

go/build and cmd/go will stop returing Targets for stdlib .a files, and
stop producing the .a files is pkg/GOOS_GOARCH. update tests to
anticipate that and to pass in importcfgs instead of expecting the
compiler can find .a files in their old locations.

Adds code to determine locations of .a files to internal/goroot. Also
adds internal/goroot to dist's bootstrap directories and changes
internal/goroot to build with a bootstrap version of Go.

Change-Id: Ie81e51105bddb3f0e374cbf47e81c23edfb67fa5
Reviewed-on: https://go-review.googlesource.com/c/go/+/442303


Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>