ensure that fully modularized build works

it must be possible to build and install each module separately, with the installation requiring elevated privileges.

to achive this, no module must reference either source or build dirs of other modules. specifically:

• no references to QT_SOURCE_TREE and QT_BUILD_TREE outside of qtbase
  • we have already done this so some degree, but the current state is unknown to me
• no references to other modules' QT.foo.sources. in fact, these variables should be scrapped
• modules must not set DESTDIR to QT.foo.plugins or QT.foo.imports where foo lives in another repository
  • i'm currently working on resolving that properly