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/py-basemap/package.py

@@ -44,7 +44,7 @@ class PyBasemap(PythonPackage):
     def setup_environment(self, spack_env, run_env):
         spack_env.set('GEOS_DIR', self.spec['geos'].prefix)
 
-    @PythonPackage.sanity_check('install')
+    @run_after('install')
     def post_install_patch(self):
         spec = self.spec
         # We are not sure if this fix is needed before Python 3.5.2.