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/superlu/package.py

@@ -48,7 +48,7 @@ class Superlu(Package):
     def install(self, spec, prefix):
         cmake_args = [
             '-Denable_blaslib=OFF',
-            '-DBLAS_blas_LIBRARY={0}'.format(spec['blas'].blas_libs.joined())
+            '-DBLAS_blas_LIBRARY={0}'.format(spec['blas'].libs.joined())
         ]
 
         if '+fpic' in spec:
@@ -76,7 +76,7 @@ def install(self, spec, prefix):
             'SUPERLULIB = $(SuperLUroot)/lib/libsuperlu_{0}.a' \
             .format(self.spec.version),
             'BLASDEF    = -DUSE_VENDOR_BLAS',
-            'BLASLIB    = {0}'.format(spec['blas'].blas_libs.ld_flags),
+            'BLASLIB    = {0}'.format(spec['blas'].libs.ld_flags),
             # or BLASLIB      = -L/usr/lib64 -lblas
             'TMGLIB     = libtmglib.a',
             'LIBS       = $(SUPERLULIB) $(BLASLIB)',