Create golang library for validating packages against specs

[ycombinator]

Create a golang library (go module) that provides an API for validating a package against the appropriate specifications. The library will need to bundle all specification versions with it.

• Foundation: Golang library for validation: foundation #12
• Folder structure validation: Golang library for validation: folder structure validation #14
• File contents validation Validate content of folder items #41

[ycombinator]

@mtojek I think the last item in the checklist is done now? If so, could you link to your PR that resolved it, check it off, and close this issue? Thanks!