convert {apkbuild, gem, python}: allow File/URI from the arg

[Dentrax]

All the melange convert subcommands only supports passing package name from the arg. It'd be great to allow file path and URI:

• melange convert python https://raw.githubusercontent.com/kubernetes-client/python/76ec6f5ce5572f66d82717e296480dc42584f0b7/requirements.txt
• melange convert python /path/to/my/requirements.txt

Progress

• python: convert/python: parse requirements.txt, skip if package exist on APKINDEX, rework error printing #704
• gem
• apkbuild

Same UX for apkbuild and gem subcommands. Wdyt?