use $$MODULE_BASE_OUTDIR & $$MODULE_QMAKE_OUTDIR directly.
this is a no-op, except that now module pris will be built in qtbase for
all modules when building without -prefix - which is only consistent with
all other artifacts.
Change-Id: I2965b2c7a15aa9e82ba6017f5f4c0daa14b6e6fe
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>