Report dependencies not found via mappings available to FawltyDeps

[Nour-Mws]

We have so far allowed identity mapping to be a last resort for when other mappings fail to find a declared package.

Note the liberal use of mappings with an s here, as in reality we still allow the resolver to consult exactly one mapping before resorting to identity mapping.
That might change with #256, which is a first step towards formalizing the mapping strategy of the resolver.

In the linked issue, I have mentioned making the fallback to identity mapping suppressable by the user.
If that's implemented, dependencies that have not been mapped could be identified by e.g. a "None" mapping type in the Json output. They would however still need to be adequately reported in the human readable output. We can possibly adapt the message we're already logging to report that a given dependency was not found in the current environment and that we're assuming it can be imported under the same package name.

[Nour-Mws]

We currently have 2 mutually exclusive "sinks":

• install deps: if a package is not found on PyPI, we have a real error thrown because of unresolved residual dependencies.
• identity mapping: every residual package is resolved. No errors thrown.

This will be the behavior when #299 is closed, but I'm closing this now as it's redundant.