go-module-fem.eclass: Add new eclass to support vendored dependency tarballs

This MR adds a new eclass go-module-fem.eclass and migrates packages to use it instead of go-module.eclass. This allows for easy usage of dependency tarballs vendored by FeM and additionally adds src_compile and src_install functions for easy installation of simple Go packages, reducing the overall size and complexity of Go ebuilds.

Closes: #109 (closed)