build systems: simpler, clearer decorators: run_after, run_before (#2860)

* PackageMeta: `run_before` is an alias of `precondition`, `run_after` an alias of `sanity_check`

* PackageMeta: removed `precondition` and `sanity_check`

* PackageMeta: decorators are now free-standing

* package: modified/added docstrings. Fixed the semantics of `on_package_attributes`.

* package: added unit test assertion as side effects of install

* build_systems: factored build-time test running into base class

* r: updated decorators in package.py

* docs: updated decorator names

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

@@ -86,7 +86,7 @@ def setup_dependent_package(self, module, dep_spec):
             join_path(self.prefix.lib, 'libmpi.{0}'.format(dso_suffix))
         ]
 
-    @AutotoolsPackage.precondition('autoreconf')
+    @run_before('autoreconf')
     def die_without_fortran(self):
         # Until we can pass variants such as +fortran through virtual
         # dependencies depends_on('mpi'), require Fortran compiler to
@@ -106,7 +106,7 @@ def configure_args(self):
             '--{0}-ibverbs'.format('with' if '+verbs' in spec else 'without')
         ]
 
-    @AutotoolsPackage.sanity_check('install')
+    @run_after('install')
     def filter_compilers(self):
         """Run after install to make the MPI compilers use the
         compilers that Spack built the package with.