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/h5hut/package.py

@@ -47,7 +47,7 @@ class H5hut(AutotoolsPackage):
     # install: .libs/libH5hut.a: No such file or directory
     parallel = False
 
-    @AutotoolsPackage.precondition('configure')
+    @run_before('configure')
     def validate(self):
         """Checks if Fortran compiler is available."""