Add some functional tests for the new resolver

Implement PipProvider

Added copyright doc

Implement resolver methods on provider interface

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.

The implementation is improved a bit to make the sorting result more
predictable for easier testing.

The weight mapping should have size `len(packages) + 1` because it needs
to contain a "sentinel" node (None).

This is using a temporary PipProvider class, which should be replaced by
the actual implementation after it is merged.

Miscellanous Documentation-related changes

Vendor ResolveLib from Git

We are vendoring from the Git source for now, so the bug fix turnover
can be quicker if there's anything wrong in the resolution logic.

HEAD up-to-date as of 2020-03-12.

Rename pep425tags -> utils.compatibility_tags

Implement the new resolver's Requirement class

Add --unstable-feature=resolver

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.

cli: revert format() related changes

Since the progress bar API is % related, it seems simpler to stick with
it.

Convert the remaining '%' formatters to '.format'.