`debug` should determine runtime from source

We should be able to walk an artifact's source to determine the runtime type. This would remove the need for GO* environment variables and more.

There may be a chance of false positives, such as projects that use scripts as part of their builds. The explicit runtime mechanism #2203 might handle that situation.

This might be best developed as a distinct library that could also be used as a command-line tool. It could be useful for IDEs for detecting different project roots. Ideally we'd be able to leverage (or extract this from) `skaffold init`.