cmd/go: change default of GO111MODULE from 'auto' to 'on'

[bcmills]

As noted in https://blog.golang.org/modules2019, “Our aim is for Go 1.13, scheduled for August 2019, to enable module mode by default (that is, to change the default from auto to on) and deprecate GOPATH mode.”

This issue tracks the first part of that goal: changing the default to on.

[bcmills]

CC @rsc @jayconrod

We will want to do #29433 in conjunction with this change. Otherwise it's way too easy to accidentally create a $GOROOT/go.mod by running a Go command anywhere in $GOROOT that doesn't contain a package.

[bcmills]

A slight wrinkle: some of the tests in misc/cgo/test use relative imports.

Some options:

• We could add a go.mod file somewhere in misc, misc/cgo, or misc/cgo/test and convert them to absolute imports.
  • That would break misc/cgo/test in GOPATH mode, since those packages are in neither GOROOT/src nor GOPATH/src.
    • We could add a special-case in the GOPATH-mode importer to fix them, but I hesitate to touch GOPATH mode more than we have to at this point.
• We could add support for relative imports in module mode.
  • We've explicitly rejected that in the past (cmd/go: 'cannot find module for path' when importing from subdirectories #26645 (comment)).
• We could keep those tests in GOPATH mode for the foreseeable future.
  • It seems a bit hypocritical to deprecate GOPATH mode without a plan to stop using it ourselves.

[gopherbot]

Change https://golang.org/cl/162697 mentions this issue: all: set GO111MODULE=off explicitly in tests that assume GOPATH mode

[gopherbot]

Change https://golang.org/cl/162698 mentions this issue: cmd/go: change the default value of GO111MODULE to 'on'

[gopherbot]

Change https://golang.org/cl/162699 mentions this issue: cmd/go: only generate a go.mod file during 'go mod init'

[bcmills]

Blocked on #26924. Let's shave this yak! 🐃

[bcmills]

I talked with @rsc, and our plan for misc/cgo/test is to remove the relative imports, add a go.mod file, and let them break in GOPATH mode.

[gopherbot]

Change https://golang.org/cl/162837 mentions this issue: runtime: make tests that invoke 'go build' module-agnostic