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>

When 'go mod init' is run in an existing project, it may import
requirements from a vendor configuration file, but the requirements
may not be complete, and go.sum won't contain sums for module
zips. With -mod=readonly, the next build command is likely to fail.

'go mod init' will now suggest running 'go mod tidy' if there are .go
files or subdirectories in the current directory.

We could potentially run 'go mod tidy' automatically within
'go mod init', but it seems better to guide users to using 'go mod tidy'
as a separate command to fix missing dependencies.

For #41712
Updates #40278

Change-Id: Iaece607f291244588a732ef4c5d576108965ca91
Reviewed-on: https://go-review.googlesource.com/c/go/+/264622
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

modload.Init now sets the default value for -mod if it wasn't set
explicitly. This happens before go.mod is loaded, so
modload.LoadModFile sets the default value again in order to enable
automatic vendoring.

Previously, cfg.BuildMod wasn't set at all if LoadModFile wasn't
called, as is the case for commands that run outside of a module
root. This problem only affected 'go install pkg@version' since other
commands are either forbidden in module mode or run with -mod=mod
(like 'go get' and 'go mod' subcommands).

This change also suppresses "missing sum" errors when -mod=readonly is
enabled and there is no module root.

Fixes #43278
Related #40278

Change-Id: I6071cc42bc5e24d0d7e84556e5bfd8e368e0019d
Reviewed-on: https://go-review.googlesource.com/c/go/+/279490
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>