Fix source mode to not trigger go modules (#23)
Fix source mode to not trigger go modules (#23)

Fixes https://github.com/jmhodges/bazel_gomock/issues/17. Investigating a bit, golang's module mode seems to trigger when using source mode with a fake  module file, resulting in downloading modules  while doing codegen. This can be avoided by prefixing the path passed  to `-source` with `$GOPATH`, so `go/packages` recognizes the source file as being in the GOPATH.

This also removes the `_go_tool_run_shell_stdout`, which  was only  called from one place. Seemed better to remove as compared having to pass the gopath around to prefix the source flag.
jmhodges/bazel_gomock,jmhodges/bazel_gomock