Fix MODULES_PACKAGE_DEPENDS_DIRS clobbered by MITKConfig.cmake

Correct behavior is to append any existing MODULES_PACKAGE_DEPENDS_DIRS
value (using list semanatics).

Signed-off-by: Taylor Braun-Jones <taylor.braun-jones@ge.com>
Check if MODULES_PACKAGE_DEPENDS_DIRS is not empty.
Ensure that project-specific package dirs are searched first.
Merge branch 'bug-18119-dont-clobber-modules-package-depends'