Simplify build system conflict checks

- removes redundant code for conflict checks
- updates and expands original check to report fatal error instead of ignoring
  files
- removes obsolete DetectAndRemovePackageHeader and RemovePackageHeader utility
  functions which are no longer needed
- adds utility function DetectBuildSystemConflict, which loops over files and
  reports an error if they exist in the lammps src directory.
- updates definition of LAMMPS_SOURCE_DIR, LAMMPS_LIB_SOURCE_DIR and
  LAMMPS_LIB_BINARY_DIR to be absolute paths. This improves instructions in
  error messages