don't process .prl files recursively

they are self-contained, as they are the result of another project's
full resolution. consequently, recursing them just burns cycles, and
additionally introduces the risk of an endless loop if the file is
botched.

Task-number: QTBUG-12711
Change-Id: I401ee691c170092cc61fe05538cec4272ed8f922
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>