Add integration with ApacheSolr for facet-based project browsing

As mentioned at http://drupal.org/node/366562#comment-1230834 -- part of why core search sucks so bad is that if you type "views" you don't get http://drupal.org/project/views as the first hit. Can't we just add an "ssfield_project_shortname" field to the solr document, and give that a relevancy factor of 1000 or something? I guess we should use both the shortname and the project title. Seems like having project_title separate from regular node titles is good, since we'd only want to outrageously boost relevancy on title matches if the word appears in project node titles. Matching "views" in the titles of issues or forum posts obviously wouldn't help this problem.

Committed code to add "ssfield_project_uri" to the solr document to HEAD. the separate title field is redundant with the proposed boost for node types: #365901: Add a bias on node type (and node-type exclusion) so I left that out.

Related issues:
#378830: Have core version facet impact the active/available sorts
#394352: When a text keyword is present, sort by relevancy by default
#394342: Port to the "sort_title" Solr schema
#394386: Add code to mark project nodes to be reindexed to solr at the right times
#377456: Add a "recent activity" sort next to the existing "latest (official) release" sort

See also #375341: use TermsCompponent for autocomplete or a directory?

#375341 isn't a blocker on this at all. The initial request here (to add ApacheSolr integration) is done. Other features/bugs should come in separate issues.