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/arpack-ng/package.py

@@ -88,8 +88,8 @@ def install(self, spec, prefix):
         options.append('-DCMAKE_INSTALL_NAME_DIR:PATH=%s/lib' % prefix)
 
         # Make sure we use Spack's blas/lapack:
-        lapack_libs = spec['lapack'].lapack_libs.joined(';')
-        blas_libs = spec['blas'].blas_libs.joined(';')
+        lapack_libs = spec['lapack'].libs.joined(';')
+        blas_libs = spec['blas'].libs.joined(';')
 
         options.extend([
             '-DLAPACK_FOUND=true',
@@ -129,8 +129,8 @@ def install(self, spec, prefix):
             ])
 
         options.extend([
-            '--with-blas={0}'.format(spec['blas'].blas_libs.ld_flags),
-            '--with-lapack={0}'.format(spec['lapack'].lapack_libs.ld_flags)
+            '--with-blas={0}'.format(spec['blas'].libs.ld_flags),
+            '--with-lapack={0}'.format(spec['lapack'].libs.ld_flags)
         ])
         if '+shared' not in spec:
             options.append('--enable-shared=no')