dependencies.py: replace hand rolled depcache with functools.cache

functools.cache caches the output of functions "automatically" without
requiring any manual management of a cache value. When used on class
methods, the cache is associated with each class instance and only
lives as long as the instance does.

The Dependencies.graph_reverse_depends method used a dict to cache
the output from pkg.get_all_depends. The get_all_depends method involves calling
portage's aux_get and parsing the DEPEND string that is returned by
it. This dict has been removed and replaced with functools.cache.

The graph_reverse_depends method did not cache the output of the
"raw=True" get_all_depends calls. This "raw" output is the literal string value for
the pkgdeps *DEPEND variables as returned by aux_get. Searching this for a
category/package sub-string allows quickly ruling out non-matching
pkgdeps, which allows skipping parsing the DEPEND string into a list
of Atoms.

Using functools cache the method that fetches the raw DEPEND string
massively improves performance for graph_reverse_depends when
searching for indirect reverse dependencies. "equery depends --indirect emacs"
runtime is ~2s with the raw value being cached, and ~60s
without. Searching for only direct reverse dependencies does not
utilize the cache and does not see any chance in runtime for me.

Signed-off-by: John Turner <jturner.usa@gmail.com>
Closes: https://github.com/gentoo/gentoolkit/pull/44
Signed-off-by: Sam James <sam@gentoo.org>

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