BPathFinder: Fix resolvable expression support

We need to use the package's .self symlink, since that does point to
the package's installation location. The containing directory lives in
/boot/system/package-links and would therefore always resolve to the
system installation location.
makefile engine: Improve template makefile

Use findpaths to find the makefile-engine to include. This addresses the
remaining aspect of #10011. I.e. the package can now also be installed
in home.