Prime the duplicate Pod cache the first time is_duplicate_pod() is called.

Prime the duplicate Pod cache the first time is_duplicate_pod() is called.

Previously it was primed when get_pod_metadata() was called. This removes the
undocumented assumption that is_duplicate_pod() will only be called after
get_pod_metadata(), and avoids reading 14 Pod files to calculate their MD5s
unless actually necessary.

This change means that the array references in @{$state{master}} are being
accessed much later during runtime. This reveals that podset() in buildtoc
had been clobbering its callers $_, which happened to be an alias to the
current element of @{$state{master}}.