cmd/go: only generate a go.mod file during 'go mod init'

In the general case, we do not know the correct module path for a new
module unless we have checked its VCS tags for a major version. If we
do not know the correct path, then we should not synthesize a go.mod
file automatically from it.

On the other hand, we don't want to run VCS commands in the working
directory without an explicit request by the user to do so: 'go mod
init' can reasonably invoke a VCS command, but 'go build' should not.

Therefore, we should only create a go.mod file during 'go mod init'.

This change removes the previous behavior of synthesizing a file
automatically, and instead suggests a command that the user can opt to
run explicitly.

Updates #29433
Updates #27009
Updates #30228

Change-Id: I8c4554969db17156e97428df220b129a4d361040
Reviewed-on: https://go-review.googlesource.com/c/162699
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
14 files changed
tree: 0db5138f041fb3adcb3257a23666b598a5436180
  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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