pacman: rewrite with a cache to speed up execution

- Use a cache (or inventory) to speed up lookups of:
  - installed packages and groups
  - available packages and groups
  - upgradable packages
- Call pacman with the list of pkgs instead of one call per package (for
  installations, upgrades and removals)
- Use pacman [--sync|--upgrade] --print-format [...] to gather list of
  changes. Parsing that instead of the regular output of pacman, which
  is error prone and can be changed by user configuration.
  This can introduce a TOCTOU problem but unless something else calls
  pacman between the invocations, it shouldn't be a concern.
- Given the above, "check mode" code is within the function that would
  carry out the actual operation. This should make it harder for the
  check code and the "real code" to diverge.
- Support for specifying alternate package name formats is a bit more
  robust. pacman is used to extract the name of the package when the
  specified package is a file or a URL.
  The "<repo>/<pkgname>" format is also supported.

For "state: latest" with a list of ~35 pkgs, this module is about 5
times faster than the original.