Draw SRU search indexes from config.metabib_search_alias

Draw SRU search indexes from config.metabib_search_alias

We had hardcoded search indexes for SRU, but thanks to the work
of Mike Rylander, it is fairly easy to remove that brittle code
and replace it with code that polls the contents of
config.metabib_search_alias and dumps it into the explain document.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@19564 dcc99617-32d9-48b4-a31d-7c20da2025e4