Packages in vendor/ directories have a "vendor/" path prefix in GOPATH
mode, but intentionally do not in module mode. Since the import path
is embedded in the compiled output, changing that path invalidates
cache entries and causes cmd/go to try to rebuild (and reinstall) the
vendored libraries, which will fail if the directory containing those
libraries is read-only.

If I understood correctly, this is the approach Russ suggested as an
alternative to https://golang.org/cl/136138.

Fixes #27285
Fixes #26988

Change-Id: I8a2507fa892b84cde0a803aaa79e460723da572b
Reviewed-on: https://go-review.googlesource.com/c/147443
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>