We're already moving them, we can remove the awkward dot that hides
them, and while we're doing that remove the useless $USER as well.
Most systems will have only one of these files - it's rare to run a
second package manager (especially for anything more than
bootstrapping a container).
This can be used to print the modification time, like `stat` with some
options.
The reason is that `stat` has caused us a number of portability
headaches:
1. It's not available everywhere by default
2. The versions are quite different
For instance, with GNU stat it's `stat -c '%Y'`, with macOS it's `stat
-f %m`.
So now checking a cache file can be done just with builtins.
Of note: The rpm/yum thing seems to be coupled, so I put it into one
function that tries the yum helper and uses the rpm path otherwise.
Zypper is already its own thing, so this should only be used for yum
and probably dnf (does that still have the helper?)
Zypper can be dropped, as that already used a separate function in the file.
Apk can just be inlined - it's literally one line for installed and another for all packages.
