Keep track of whether the -mod flag was set explicitly. When
-mod=readonly is the default, we'll want to adjust our error messages
if it's set explicitly.
Also, register the -mod, -modcacherw, and -modfile flags in functions
in internal/base instead of internal/work. 'go mod' commands that
don't load packages shouldn't depend on internal/work.
For #40728
Change-Id: I272aea9e19908ba37e151baac4ea8630e90f241f
Reviewed-on: https://go-review.googlesource.com/c/go/+/253744
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com>