cmd/go: add modload.NeedRoot mode for commands that need module root

This makes error reporting a bit more consistent for 'go mod'
subcommands. Most of these commands only work in module mode when a
go.mod file is present.

Setting modload.ForceUseModules reports an error when GO111MODULE=off.

Setting modload.RootMode to modload.NeedRoot reports an error when no
go.mod file is present.

Change-Id: I1daa8d2971cb8658e0c804765839d903734a412e
Reviewed-on: https://go-review.googlesource.com/c/go/+/254369
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
10 files changed
tree: 6f9fc4493cb0b775dabd1affa30d6e4209b153c3
  1. .gitattributes
  2. .github/
  3. .gitignore
  4. AUTHORS
  5. CONTRIBUTING.md
  6. CONTRIBUTORS
  7. LICENSE
  8. PATENTS
  9. README.md
  10. SECURITY.md
  11. api/
  12. doc/
  13. favicon.ico
  14. lib/
  15. misc/
  16. robots.txt
  17. src/
  18. test/
README.md

The Go Programming Language

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