Change multimethods to take first match instead of being rigid.

- Formerly required that one and only one spec match
- Now allows first match in a list (more flexible and more intuitive)

- introduces new bug that provides() doesn't do the correct thing
  when a version is not in a range that has been explicitly provided.
- TODO: fix this.

lib/spack/docs/developer_guide.rst

@@ -3,6 +3,8 @@
 Developer Guide
 =====================
 
+This guide is intended for people who want to work on Spack's inner
+workings.  Right now it's pretty sparse.
 
 Spec objects
 -------------------------

lib/spack/docs/packaging_guide.rst

@@ -1060,17 +1060,17 @@ for example:
            # the default, called when no @when specs match
            pass
 
-       @when('mpi@3:')
+       @when('^mpi@3:')
        def setup_mpi(self):
            # this will be called when mpi is version 3 or higher
            pass
 
-       @when('mpi@2:')
+       @when('^mpi@2:')
        def setup_mpi(self):
            # this will be called when mpi is version 2 or higher
            pass
 
-       @when('mpi@1:')
+       @when('^mpi@1:')
        def setup_mpi(self):
            # this will be called when mpi is version 1 or higher
            pass