cmd/go: mod init outside of GOPATH silently fails

Running `go mod init` outside of GOPATH with `GO111MODULE=off`
silently fails. This behavior was undocumented.

This CL makes go mod fail with the error:

   go: modules disabled by GO111MODULE=off; see 'go help modules'
Comparing with already erroring GO111MODULE=<value> conditions:

* With GO111MODULE=auto, inside GOPATH:
    go modules disabled inside GOPATH/src by GO111MODULE=auto; see 'go help modules'
* With GO111MODULE=auto outside of GOPATH:
    go: cannot determine module path for source directory /path/to/dir (outside GOPATH, no import comments)

Fixes #31342

Change-Id: I749787d2a8640913c4ac263072d051314d76e778
GitHub-Last-Rev: b38447457d8cabed367ea4872cf7f238a49539c7
GitHub-Pull-Request: golang/go#31255
Reviewed-on: https://go-review.googlesource.com/c/go/+/170697
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed
tree: 6fea5a7fe48f7515b29356148735ed78d8c3c41a
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
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