command-line tool or library or both?

fontmake is primarily a CLI tool, not a library. But with Python, the distinction is not that strict and people can and do try to import fontmake and use it like any another module.

E.g. see https://github.com/googlefonts/gftools/pull/278/files#r541051261

We should decide to either explicitly support that use case (e.g. by removing the nasty logging setup in FontProject class), or discourage/disallow this more clearly. The current grey area where things stand at the moment is not very helpful.