python3: avoid unnecessary rebuilds

python3: avoid unnecessary rebuilds

Move the order in which BuildPackage is called, so that the libpython
package is built ahead of the module packages, to avoid forcing a
clean-build of the package when 'make package/python3/compile' is called
a second time without changes.

The library must be built first, so that when the buildsystem checks for
ABI version changes using libpython3.version, its timestamp should be
older than the dependent package's STAMP_PREPARED file.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>