ResolveLib *should* always produce a graph with only one root vertice,
which is None. But we don't really need to rely on that implementation
detail. Vertices without any parents can be assigned 0 in all cases.
This introduces a new general option --unstable-feature that can be used
to opt into "preview" features in pip not enabled by default. Currently
the only available feature is "resolver".
A stub resolver interface (which would fail on invocation) is provided
to respond to the flag.
The --unstable-feature option is hidden from --help since the resolver
does not yet work. This suppression should be removed when we release
the resolver for general/public testing.
This gives us a concrete place to put the new resolver code and
resolver-specific modules (`resolution.resolver`).
The reason for another level of hierarchy compared to other modules
is to allow us to move other modules here as they
become implementation details of the legacy resolver. Examples I
have in mind are: `req.req_set`, `req.req_install`,
`req.constructors`, and `operations.prepare`.
