relocate_install_names: rewrite cellar names before prefix names

The cellar may overlap with the prefix, so if we replace the prefix
first, we will end up with paths like "@@HOMEBREW_PREFIX@@/Cellar"
instead of "@@HOMEBREW_CELLAR@@", which will break on installations
where the cellar and prefix are disjoint.
Insert placeholders for prefix and cellar in relocatable bottles
Do relocation check while preparing bottle
Remove special handling for relocatable pkgconfig files
Teach installer about prefix and cellar placeholders
Pull prefix and cellar placeholders into constants