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/openmpi/package.py

@@ -145,7 +145,7 @@ def verbs(self):
         elif self.spec.satisfies('@1.7:'):
             return 'verbs'
 
-    @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
@@ -239,7 +239,7 @@ def configure_args(self):
 
         return config_args
 
-    @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.