Moby is not a Go module; to prevent anyone from mistakenly trying to
convert it to one before we are ready, introduce a check (usable in CI
and locally) for a go.mod file.
This is preferable to trying to .gitignore the file as we can ensure
that a mistakenly created go.mod is surfaced by Git-based tooling and is
less likely to surprise a contributor.
Signed-off-by: Bjorn Neergaard <bneergaard@mirantis.com>
