Use IF(NOT TARGET ...) before creating imported targets.

Use IF(NOT TARGET ...) before creating imported targets.

Initially we didn't do this because someone could accidentally create
another target of a conflicting name, and used a variable to store whether we
have created the target already or not.

That wasn't adequeate to deal with finding the package in a scope
like a function, so we used a directory property. However, the directory
property is not valid in the same scopes as the defined target. For
example, finding a Qt module in both a directory and a subdirectory causes
a conflict.

As it is already unlikely that a target would be accidentally created with
a name like Qt5::Core, we should simply use the IF(TARGET) form.

Change-Id: If64f25d45f51edcd1edb0d4bfb5ed3bb2479bd27
Reviewed-by: David Faure <faure@kde.org>