Pull facet blocks out of apachesolr_search.module (use facet_api)

Basically, this is just moving code around and renaming some functions. No changes made to the logic.
One function was moved to apachesolr.module instead of the new apachesolr_facets.module because apachesolr_search.module also needed it.

With this patch, I was able to disable apachesolr_search.module and get facet blocks with an apachesolr_views view.

This would also be a first step to some refactorings that I am planning and that should lead to some cool stuff (I'm still working on my "multiple search environments" thing. :-)).

My main idea is to move re-usable code out from the apachesolr_search module to allow other modules to use it. If that code is placed in a .inc file, then other modules that might need that code won't be able to leverage Drupal's dependencies system. Ultimately, following this idea means that apachesolr_search would become quite small, only containing code that really depends on search.module.

Although the above patch relates only to facet-related code, perhaps "apachesolr_facets" is not the right name for a module whose general goal would actually be to re-use some integration code that might be too specific for apachesolr.module, but at the same time is too generic to be in a module that depends on core search.

I still think that this would be a step in the right direction, that is towards more modularity and a better structured API, but it looks like I have not been able to convince the module's maintainers.