go/build: bypass importGo if srcDir is in GOROOT/src

This fixes the builder flake observed in
https://build.golang.org/log/84fe80f8f091b9cef639b3ae2422a673f1462810,
which could be replicated by running

	GOPROXY=off GOPATH=$(mktemp -d) go test go/internal/srcimporter

Updates #30228
Fixes #30760

Change-Id: Ibf8b7a2e211611960b074b74af91acd4f0196edb
Reviewed-on: https://go-review.googlesource.com/c/go/+/166977


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>