New interface for passing build information among specs (#1875)

- Added a new interface for Specs to pass build information
  - Calls forwarded from Spec to Package are now explicit
  - Added descriptor within Spec to manage forwarding
  - Added state in Spec to maintain query information
  - Modified a few packages (the one involved in spack install pexsi) to showcase changes

- This uses an object wrapper to `spec` to implement the `libs` sub-calls.
  - wrapper is returned from `__getitem__` only if spec is concrete
  - allows packagers to access build information easily

var/spack/repos/builtin/packages/cp2k/package.py

@@ -88,12 +88,10 @@ def install(self, spec, prefix):
             cppflags = [
                 '-D__FFTW3',
                 '-D__LIBINT',
-                '-I' + spec['fftw'].prefix.include
+                spec['fftw'].cppflags
             ]
             fcflags = copy.deepcopy(optflags[self.spec.compiler.name])
-            fcflags.extend([
-                '-I' + spec['fftw'].prefix.include
-            ])
+            fcflags.append(spec['fftw'].cppflags)
             fftw = find_libraries(['libfftw3'], root=spec['fftw'].prefix.lib)
             ldflags = [fftw.search_flags]
             libs = [
@@ -154,15 +152,17 @@ def install(self, spec, prefix):
                     '-D__SCALAPACK'
                 ])
                 fcflags.extend([
+                    # spec['elpa:fortran'].cppflags
                     '-I' + join_path(
                         spec['elpa'].prefix,
                         'include',
                         'elpa-{0}'.format(str(spec['elpa'].version)),
                         'modules'
                     ),
+                    # spec[pexsi:fortran].cppflags
                     '-I' + join_path(spec['pexsi'].prefix, 'fortran')
                 ])
-                scalapack = spec['scalapack'].scalapack_libs
+                scalapack = spec['scalapack'].libs
                 ldflags.append(scalapack.search_flags)
                 libs.extend([
                     join_path(spec['elpa'].prefix.lib,
@@ -184,8 +184,8 @@ def install(self, spec, prefix):
                 libs.extend(self.spec['mpi'].mpicxx_shared_libs)
                 libs.extend(self.compiler.stdcxx_libs)
             # LAPACK / BLAS
-            lapack = spec['lapack'].lapack_libs
-            blas = spec['blas'].blas_libs
+            lapack = spec['lapack'].libs
+            blas = spec['blas'].libs
 
             ldflags.append((lapack + blas).search_flags)
             libs.extend([str(x) for x in (fftw, lapack, blas)])