REUSE metadata traversal

[ForNeVeR]

There should be a command in format:

$ dotnet authors --source reuse --target json --output authors.json

(other potential --source in the future will be git, and other potential --target will be reuse)

Which should collect the copyright holders from the REUSE files according to the config (if passed) and produce the authors.json file.

Resulting data format example:

{ "authors": [{
  "name": "Friedrich von Never",
  "contact": [{
    "url": "mailto:friedrich@fornever.me"
  }, {
    "info": "any optional text info"
  }]
]}}

contact may consist of different entries in case there are different copyright entries in different files.

URLs should be valid (thus dotnet-authors should recognize at least mailto: and https:// correctly when needed).

In the future, config will help to deduplicate authors.

There's nothing wrong in exposing the author info this way since they are stored in the REUSEable sources anyway.