Add golang support

[wbxyz]

This still has a handful of polishing task required before it works:

• Decide on the right string literal for ecosystem, and the right package name (go vs golang vs goproxy)
• Clean up the output path logic. I was using docker build locally, and that's hardcoding the /src directory as a prefix on the output path
• Find a cleaner way of calling mod/zip package. There's no CLI version I've found so I'm writing a tiny main.go file and running it
• Actually use the binary that was built during deps
• Make sure packages that have git repo redirects (like the golang.org/x/ packages) are supported correctly
• Use canonicallization on the git repos
• Make sure packages in subdir work
• Clean up the version parsing in the tags
• Verify that comparison and GCB work (I only tested using docker locally)