Enhance go project detection by referring to go.mod file

[rudle]

This PR fixes an issue encountered when working in a go project that doesn't have any .go files at the root. The fix is to check for a go.mod file at the root of the project, avoiding a recursive descent of the project. The go.mod file is new-ish and will be present in projects that are using Go modules. The file will be present in all (or almost all) Go projects when Go 1.13 is released later this year.

I didn't add any tests, but this is the fix I applied to my local install of projectile and it worked great.

Feel free to remove or tweak my Changelog entry, this change may not be big enough to warrant documentation.

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (make test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)

[rudle]

Thanks for the review. Feedback and merge conflicts have been addressed.

[bbatsov]

Thanks!