Rename all the spec tests to follow the rspec convention of *_spec.rb rather
than unadorned *.rb; this also makes it easier to work with them consistently
without using the Rakefile support that customizes that.

Because we pull in spec_helper in individual tests, we need to use a
consistent path or Ruby will evaluate it multiple times.  Make the path
consistent by expanding it before require.

Facter no longer knows anything about Puppet, so there's
no inter-dependency issue.

I was previously creating a mock Puppet class, and this
actually uses Puppet's code.

Note that this use of Puppet creates some additional ordering
problems, which I'm going to fix in the next commit by
undoing all of this work. :)

This should make it possible to move all facts to separate
files and only load them on demand.

This makes it easier for our loader to distinguish between
code that Facter uses and new facts.

There's now no @@facts instance variable; instead, there's
a collection, and it's responsible for keeping references
to all of the facts.

All of the old interface methods just delegate to the collection.