Make index.pidx mandatory in init command

[chaws]

Issue extracted from #50 (comment)

a) let's make index-url mandatory - where index-url may point to a local index.pidx file being e.g. generated using vidx2pidx or a a schema compliant file without entries

[chaws]

@jkrech is it a valid use case to allow local index.pidx files as index-url?

Ex: cpackget init --pack-root packroot/ path/to/my/index.pidx

[jkrech]

@chaws I believe this is a valid use case, even though we are not supporting it today in MDK and ArmDS as they use the fixed download location. However I think that people could use the vidx2pidx utility to create their own set of "public" packs or even extend a public index with a company internal pack index.