In pod_lib.pl's get_pod_metadata(), generate lookup hashes directly.

In pod_lib.pl's get_pod_metadata(), generate lookup hashes directly.

Previously the code was structured to build arrays for various categories of
files, then generate hashes from those arrays. However, the arrays had no
other purpose, and the values in the hashes were only needed for truth tests,
so it's better to generate the lookup hashes directly.

@disk_pods hasn't actually been used since commit d5e2eea989a69524 refactored
pod/buildtoc to no longer chdir to the pod/ directory.