cmd/go: add support for 'go run pkg' or 'go run .'
To date, go run has required a list of .go files.
This CL allows in place of that list a single import path
or a directory name or a pattern matching a single patckage.
This allows 'go run pkg' or 'go run dir', most importantly 'go run .'.
The discussion in #22726 gives more motivation.
The basic idea is that you can already run 'go test .'
but if you're developing a command it's pretty awkward
to iterate at the same speed. This lets you do that,
by using 'go run . [args]'.