[BUILD] fix build system bug

- use OpenMSBuildSystem_externalLibs.cmake on top level to find correct
  include paths not only for OpenMS but also for OpenSwathAlgo and other
  libraries
- move the part of OpenMSBuildSystem_externalLibs.cmake specific to the
  openms-core (responsible for finding the actual library) to
  src/openms/cmake_findExternalLibs.cmake
- use OPENMS_HOST_DIRECTORY instead of PROJECT_SOURCE_DIR to determine
  correct location of contrib

fixes #699
[BUILD] rename file, fix comment
Merge pull request #708 from hroest/fix/fix_buildsystem_path

[BUILD] fix build system bug related to contrib finding.