Introduce 'requires'

[sdboyer]

This introduces the notion of a 'required' package:

• Required packages must be present in any complete solution returned by the solver.
• Requiring a package that is a logical child of the project root is not an error, but it is a no-op.
• Requiring and ignoring the same package from a manifest is an error.

This is a root-only capability: listing required packages is, thus, a RootManifest method.

• Base type updates - interface and structs
• Set of bimodal tests
• Prepare() validation tests
• Add/update fail types as needed
• New logic in the solver itself
• Docs

Addresses #42

[sdboyer]

This brings us back to another variant of a familiar question - if an unimported dep is constrained in the manifest, does that constraint still come into effect?

I think it's pretty clear in this case, though: having the requires list is basically equivalent to having an import path, and these are both emerging from the same manifest (so we can assume the same user has control over it), thus the constraint should be applied. And because deps can't express requires, we don't have the same transitivity problem as with normal imports.

[codecov-io]

Current coverage is 76.92% (diff: 82.75%)

Merging #114 into master will increase coverage by 0.13%

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+ Hits           2611       2637    +26   
- Misses          599        600     +1   
- Partials        190        191     +1   

Powered by Codecov. Last update 71648f3...7adc457

[sdboyer]

This doesn't introduce any new failure types, or change any of the existing ones. That means we're leaving it on the tool to figure out if a given package-related failure is due to a real import, or a require.

This is fine for now, though in the future if we end up wanting to do any kind of lineage tracking, we might need more.