Improve performance in pmb.helpers.pmaports find package

I've noticed that finding packages in our pmaports folder got very slow lately, so I decided to investigate.

For analyzing I've added a global variable to sum all the time spent in the find method of pmb.helpers.pmaports.py, which is called many times during pmb.chroot.apk.install()

I've just used the pmbootstrap qemu command which installs all the required packages before starting (device: qemu-amd64, ui: gnome)

Total time: ~17 seconds

I've separated the improvements in 3 different commits:

1. Cache the results of the _glob_apkbuilds method (with * returns the list of paths of all the packages). This reduced the total time to ~8 seconds

2. Before looking for the package APKBUILD path, check if the package exists. This reduced the total time to ~1 second

3. The last commit caches the result of the get_list method (which returns the list of packages). This reduced the total time to ~0.6 seconds

I think it's quite a big performance gain from 17 to 0.6 seconds 🎉