Introduce cmd/golist to list provided/imported packages over entire project

[x] list go files without invoking go list directly
[x] skip vendor directories
[x] skip stdlib packages and C
[x] skip all provided packages that has only files with main package
[x] list provided packages (./golist --package-path PATH --provided)
[x] list imported packages in non-test files (./golist --package-path PATH --imported)
[x] list packages imported by tests (./golist --package-path PATH --imported --tests)
[x] list packages with tests (./golist --package-path PATH --provided --tests)

gofed / symbols-extractor