add provides functionality

Snippet from https://github.com/nerdvegas/rez/issues/673:

In VFX there are often cases where a DCC ships with its own copies of other libraries (python is a classic example). Having rez resolve an environment that includes such a DCC, but also includes a standard installation of such a library, can be problematic.

To help manage this situation, it would be useful if a package could describe that it also provides another package as part of its own installation. This is another case where ephemerals come to the rescue - along with some changes to the solver.

To indicate that a package provides its own copy of another package, you would include it as a package feature:

requires = [
".{this.name}.feature.provides.python-2.7.3"
]
When the solver encounters a "provides" ephemeral, it would use it in lieu of its possibly already-resolved, real package counterpart. If a provided package ends up in the resolve, it plays no part in configuring the environment (the responsibility for that is left to the package - typically a DCC - doing the providing).

Note that the distinction between (eg) python-2.7.3 and python==2.7.3 as a provides statement is important. Whereas python-2.7.3 would take precedence over any python package within that range (including for eg python-2.7.3.1), python==2.7.3 would provide for that exact version only. If python-2.7.3.1 ended up in the resolve in this case, it would cause a conflict.

We will also need to consider what it means if >1 package provide the same package. Perhaps a configurable setting would cause a warning or error; if a warning, perhaps the first package encountered wins.

The issue names no files, tests, or entry points. Start by tracing Rez's package-feature and solver handling for ephemerals, then determine how provided packages affect resolution and environment configuration. Done means defining the precedence and conflict behavior for version ranges, exact versions, and multiple providers, and implementing the resulting behavior.

Written by the indexing model from the issue text.