Ensure source folders appear first in the classpath list.

After running the other Classpath Configuration classes the source folders
are out of sequence, causing all sorts of problems looking up source. This
is an attempt to re-order the classpaths.  It hasn't yet been tied into the
code, but here for code review and comment.
BTW m2e also does the same thing. Look at `AbstractJavaProjectConfigurator#addProjectSourceFolders`.