support so-called "splitted source directories"

     Change: splitted-source-directory
     Purpose: support so-called "splitted source directories", i.e.,
                     source files (listed on SourceX and PatchX headers)
                     can be placed into %{_specdir} in addition to
                     %{_sourcedir}.
     Reason: OpenPKG ultra-strictly separates sources files into
                     two classes: external ones -- those which are
                     referenced with fully-qualified URLs and can be
                     (re-)fetched from the Internet -- and internal
                     ones -- those which are references as plain files
                     and which are kept in the local version control
                     system (VCS). As in OpenPKG the .spec file is just
                     one of those "internal" files, they are all stored
                     under %{_specdir} while %{_sourcedir} is used
                     for everything which is downloaded from external
                     sources (in OpenPKG even an automated process).
     Hint: The even better solution would be to introduce
                     %{_sourcedir_external} and %{_sourcedir_internal}
                     and support this throughout RPM.
                     (afb) rpm4darwin also separated these two, but
                     kept the internal in SOURCES and added a new
                     directory DISTFILES for holding the external...
                     The external ones were automatically fetched
                     from a central repository over HTTP if missing.
                     (http://prdownloads.sourceforge.net/rpm4darwin/)
                     This can still be done in RPM, by setting the
                     two macros: %{_Rsourcedir} and %{_Rpatchdir}