Update remaining packages to use Stage.source_path (#11662)

#11528 updated Stage to always store a Package's source in a fixed
directory accessible via `Stage.source_path` This left behind a
number of packages which were expecting to access the source code
via `Stage.path`. This Updates those packages to use
`Stage.source_path` instead.

This also updates the name of the fixed directory: The original name
of the fixed directory was "src", so if an expanded archive created a
"src" directory, then users inspecting the directory structure could
see paths like "src/src" (which wasn't wrong but could be confusing).
Therefore this also updates the name of the fixed directory to
"spack-src".

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

@@ -241,5 +241,5 @@ def install(self, spec, prefix):
     @run_after('install')
     @on_package_attributes(run_tests=True)
     def check_build(self):
-        make('-C', join_path(self.stage.path, 'charm/tests'),
+        make('-C', join_path(self.stage.source_path, 'charm/tests'),
              'test', parallel=False)