Reporting should be possible with just the public API

[nedbat]

Is your feature request related to a problem? Please describe.
People (like coveralls) end up using internal details of coverage.py because they can't get everything they need from the public supported API. Then when the internals of coverage.py change, their code breaks.

Describe the solution you'd like
We should extend the public API to support their needs. One good litmus test would be: if coverage.py's own reporting commands (html, xml, json, report) could be written with only the public API.

[TheKevJames]

Alright, well, to get this conversation started, this stemmed from my work updating coveralls to support v5 of coverage.py. PR link. More generally, our usage of the coverage.py library is pretty much confined to this file -- note that there's a whole lot of fallback logic there for supporting v4 and v3 of coverage, which we've not yet deprecated support for.

[nedbat]

See #1273