PIP's installation procedure was too complicated, requiring obscure
postinstall and prerm scripts to move files created by setup procedure.
But since setuptools is required anyway, it's better to use a special
flag created for the purpose of package creation. Resulting directory
structure is flatter and unnecessary files that had to be moved by
prerm/postinstall scripts are not created anymore.

Signed-off-by: Jan Čermák <jan.cermak@nic.cz>