- kpmcore (when used as target "kpmcore") has an interface
include directory that does not contain the "kpmcore/"
subdirectory. But the headers it has installed, assume it
is there (e.g. kpmcore internals use #include <fs/filesystem.h>).
- add an alias at Calamares level that sticks in some more
includes, adds the relevant WITHOUT_kpmcore when it's
not there, etc.
I think we had this (kind of) module a long time ago and it was
removed for over-complicating things; re-introduce one now that
KPMcore is used in 3 different places and all would benefit
from consistent API handling / defines.
