go/build: don't check if srcDir in GOPATH when deciding to use modules

go/build: don't check if srcDir in GOPATH when deciding to use modules

go/build.Context.Import loads package information using 'go list' when
in module mode. It does this when GO111MODULE is not "off", there is a
go.mod file in any parent directory, and neither the path nor the
source directory are in GOROOT. Import no longer checks whether the
source directory is in GOPATH if GO111MODULE=auto or unset.

Also fixed subdirectory checks that did not handle relative source
directory paths. mod_gobuild_import should have failed when we changed
the meaning of GO111MODULE=auto but didn't because of this.

Fixes #32799

Change-Id: Ic5210b7e00cb58f91ea9455b67b49d5aed4eec63
Reviewed-on: https://go-review.googlesource.com/c/go/+/184098
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>