Install target adds many empty directories from the build system

The installation of the hinting files also copies many unrelated directories, e.g. */CMakefiles/ and all the other build directories below.

This is due to https://github.com/qtproject/pyside-pyside-setup/commit/14af709e10fa8a2e3c84094c13ebdda9833be124#diff-e6a90961d3665c0d196e71fcb67c8a0e

The install(DIRECTORY ... FILES_MATCHING *.pyi command does not exclude empty directories from the base directory:
https://cmake.org/cmake/help/v3.13/command/install.html#installing-directories

The FILES_MATCHING option may be given before the first match option to disable installation of files (but not directories) not matched by any expression.