cmd/go: don't import requirements into existing go.mod files
Previously, if a go.mod file was present, and it only contained a
module directive, any module-aware command would attempt to import
requirements from a vendor configuration file like Gopkg.lock.
This CL removes that functionality. It was undocumented and untested,
and it can cause problems with -mod=readonly. It should never come up
for go.mod files created with 'go mod init', since they have a "go"
directive.
For #40278
Change-Id: I64c0d67d204560aa5c775d29553883d094fd3b72
Reviewed-on: https://go-review.googlesource.com/c/go/+/264620
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>