[PATCH v4] Add package managers as a plugin

["Fredrik Gustafsson" <fredrik.gustafsson@axis.com>]

OE-core today has three different package managers, the well-known formats deb
and rpm is supported as well as ipkg that is good for embedded devices.

When building and having a good cache hit, a significant amount of time is
spent in the phase of generating a rootfs, which is really about the
performance of the package manager. To save build time and also get a
package manager that is suitanle for use on targets where flash memory is a
concern, support for apk is suggested.

However, it might or might not be what's wanted for OE-core since it increases
the test matrix. Therefore I will refactor the package management code to
allow a layer to add a new package manager without editing the meta layer.

This refactor will be divided into multiple patch series to be easier to review,
this series will only move code but keep existing functionality.