Rather than using the name of the keg for the key use the full path.
This provides several advantages:
- there's no need to invalidate the cache on a `brew upgrade` or
`brew switch`
- it's easier to figure out what cache entries can be removed and this
can be done whenever a keg is removed by `brew uninstall` or
`brew cleanup`.
Also, ensure that an `install` (or `reinstall`, `upgrade`) always
results in the cache being rebuilt for that keg (in case different
options were used).
Cache all the non-weak dynamic library links for a keg rather than the
result of running `brew linkage`. This means that we correctly handle
changes to e.g. what non-keg files are present on disk.
