Add CONTENTS indexing support for optimization of owner lookups. The

Add CONTENTS indexing support for optimization of owner lookups. The
vardbapi cache maintains a hash table (inside vdb_metadata.pickle)
that serves to index package contents by mapping the basename of file
to a list of possible packages that own it. This is used to optimize
owner lookups by narrowing the search down to a smaller number of
packages. It increases the size of vdb_metadata.pickle by approximately
30% and it's used in the following cases:

 * When an unexpected file collision occurs (whether
   or not collision-protect is enabled)

 * `emerge <filename>`

 * `portageq owners`

(trunk r10609)

svn path=/main/branches/2.1.2/; revision=10622