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/ipopt/package.py

@@ -53,8 +53,8 @@ def install(self, spec, prefix):
         mumps_flags = "-ldmumps -lmumps_common -lpord -lmpiseq"
         mumps_libcmd = "-L%s " % mumps_dir.lib + mumps_flags
 
-        blas_lib = spec['blas'].blas_libs.ld_flags
-        lapack_lib = spec['lapack'].lapack_libs.ld_flags
+        blas_lib = spec['blas'].libs.ld_flags
+        lapack_lib = spec['lapack'].libs.ld_flags
 
         configure_args = [
             "--prefix=%s" % prefix,