If GO111MODULE=on but there's no go.mod, GOMOD will be set to /dev/null
or NUL and there will be no main module. goimports should handle this
case roughly the same way the go command does.
Fixesgolang/go#30855
Change-Id: I6fbf4c056000db5abd8788a6014ae5f13b1c8cd4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/167860
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
