Handle conditions for SUBDIRS *more* correctly

There are project files that not only do SUBDIRS -= foo depending on
some condition, but also SUBDIRS += foo. To handle these cases
correctly we need to do a whole pass across all scopes and collect
all addition and subtraction conditions.

Once we have that information, we can compute a final condition for a
particular subdirectory. After we have the condition information for
all subdirectories, we can simplify all those conditions using SimPy,
and group all subdirectories that have the same simplified condition.

Finally we print out the subdirectories grouped by the simplified
condition.

The end result is similar to how we have extend_target() calls with
multiple sources grouped into a single conditional call, except in
this case it's subdirectories.

Amends b26b1455d75709a53f50e1d3d41c384f8e90b576.

Change-Id: I5b648ac67264ba9a940561baee5b49c43c13b721
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Qt CMake Build Bot

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