Put locally built packages into the cache, GH uses them

When installing packages, built source and binary packages
are added to the cache.

GitHub remotes check for the latest 1) binary, 2) source,
3) package tree archive in the cache.

With these changes switching between GH and CRAN package
versions is very quick.

If you need to re-compile/re-download a package, you can
 use the `?source`/`?nocache` parameters.

Closes https://github.com/r-lib/pak/issues/77.