Add support for golang 1.11+, go modules

[jonabc]

This PR updates the Go dependency source enumerator to better support newer versions of the Go language.

1. Uses the go list -deps option to get all dependencies in a single call rather than needing to call go list for each dependency.
2. If a go project is set up to use modules, uses the modules version number instead of a git SHA1 for version information

This ended up including a non-trivial refactor of the go dependency source to give an easy branching point for sourcing package information for older go versions vs go list -deps as described in (1) ☝️ .

[jonabc]

Uses the go list -deps option to get all dependencies in a single call rather than needing to call go list for each dependency.

In the CI environment, the go 1.11.x tests consistently run ~1minute, ~33% faster than for previous versions due to this speedup 🎉 ! And thats only for 2-3 dependencies in the test project. For go projects with a significant number of dependencies the change will be even more dramatic.