Do not load bitmap indexes during directory scans

Previously, if a bitmap index had not been loaded yet, it would get
loaded during a directory scan. Loading a bitmap file can be expensive
and there is no immediate need to do so during a scan. Fix this by
simply setting bitmap index file names on the Packs during directory
scans so that bitmaps can be lazily loaded at some later point if they
are needed.

This change has the side affect of no longer marking a Pack valid if it
is currently invalid simply because a bitmap file has been found, as
there is no valid reason to do so and this can incorrectly mark a Pack
without an index, or with other issues valid. Since the initial lack of
a bitmap file, or an invalid one, or the deletion of one, would not
result in the Pack being marked invalid, there is no need to overturn
the invalid flag when a new bitmap file is found.

Change-Id: I056acc09e7ae6a0982acd81b552d524190ebb4be
Signed-off-by: Martin Fick <mfick@nvidia.com>