Mooooooo.

Switch to using Moo. Unfortunately importing Moo unconditionally imports
strictures.pm, which I could do without: fatal warnings are really not
helpful unless I've asked for them. For now, disable it with a big
hammer; I'd like to find a smaller hammer later, but I'm not sure there
is one without patching Moo.
Extract more information from the core perl pkg.

This removes all instances of the code being specific to my setup.
There's still a bit of a hack in the case of site_bin being equal to bin
in the core Config: I can't see any way to resolve the file conflict, so
for now we make up our own site_bin directory.
Warn if we change INSTALLSITE*.

Also cache the results per-jail, so we don't have to choose again for
every build.