make .prl processing less convoluted

don't look up the files and normalize the paths multiple times, as this
is inefficient and hard to understand.
on the way, processPrlFile() got unnested, and libExists() got nuked.

note that a missing QMAKE_PRL_TARGET will be now complained about, which
really should never happen.

Change-Id: Ibcd77a7f963204c013548496ecd2d635e1a4baba
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

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