Add export-dev for exporting all libraries (see #230)

Add export-dev for exporting all libraries (see #230)

The new target export-dev builds all libraries and copies the
created .a archives and library headers to export-dev/
subdirectory.

Unlike the existing approach from tools/export.sh we do not
list the libraries manually but copy all of them automatically.

The approach relies as much on Meson to do the heavy-lifting
as possible and the invoked shell script merely copies the files.
The script honors exported DESTDIR variable if set and
ensures thin archives are converted before export.

Also helenos-pkg-config was rewritten to Python for better
maintainability.

Usage:

    ninja export-dev
    env DESTDIR=harbour-export ninja export-dev