devel/p4 devel/p4api: cache distfiles in LOCAL
Perforce unhelpfully rebuilds their distfiles every few hours, changing the
archived files' timestamps in the process. This obviously changes their
checksums. Work around the issue by caching local copies of the distfiles and
using those for the checksums.
Also, set a sane DIST_SUBDIR since the distfiles are ambiguously named.
Also, take over maintainership from gordon.