Forbid empty sourceroot and go.mod path finder

[tonyredondo]

This PR adds an additional fallback when the source root can't be detected (to improve the detection of a valid source root). This fallback is based on the path of the go.mod file, using the current folder as the starting point and going up from parent to parent folder.

According https://blog.golang.org/using-go-modules the go.mod file should be at the root of the project (in case the project is using go modules).

With this PR the source root detector has the following priorities:

1. SCOPE_SOURCE_ROOT environment variable.
2. CI environment variables.
3. .git folder root path
4. go.mod path
5. /

Note: Each value is tested using os.Stat to validate the directory existence, in case the value is set but the directory doesn't exist, then the value is invalid and the next fallback is called.