Remove the distinction between resources path and icu data path

Also, unconditionally override DIR_QT_LIBRARY_DATA to make the path
available to extensions even if we are using static or shared icu
data instead of a data file.

[ChangeLog][Behavioral Changes] Qt WebEngine will assume that the
resources and the icu data files are in the same directory when
deploying the application.

Fixes: QTBUG-74586
Change-Id: I563b74a19fbd1334358f9cfe7c1824706cdca03a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>