- add `getCallable`, `getArg` and `getNode`
- these are `none` for summary calls
- revert "external" uses (they had been changed to `DataFlowSourceCall`)
Notice the strange thing with treating `mypkg.foo(42)` as a ClassCall,
but completely ignoring `mypkg.subpkg.bar(43)` -- due to having the two
`ClassValue`s:
- `Missing module attribute mypkg.foo`
- `Missing module attribute mypkg.subpkg`
But not `Missing module attribute mypkg.subpkg` with the current import
structure.