ENH: add simple from_string method for Package class.
ENH: add dependencies/provides support to Package.from_string.
ENH: fix repr to handle dependencies/provides.
DOC: update example to use new Package.from_string.
REF+ENH: rename Policy to DefaultPolicy.

We fixed installed over non-installed precedence even if installed <
non_installed as well.
REF: consolidate code that select best candidate.
TST: remove obsolete expected failure.