This turned out to be fairly simple. Given an import such as
```python
from foo.bar.baz import quux
```
we create an API-graph node for each valid dotted prefix of
`foo.bar.baz`, i.e. `foo`, `foo.bar`, and `foo.bar.baz`. For these, we
then insert nodes in the API graph, such that `foo` steps to `foo.bar`
along an edge labeled `bar`, etc.
Finally, we only allow undotted names to hang off of the API-graph
root. Thus, `foo` will have a `moduleImport` edge off of the root, and
a `getMember` edge for `bar` (which in turn has a `getMember` edge for
`baz`).
Relative imports are explicitly ignored.
Finally, this commit also adds inline tests for a variety of ways of
importing modules, including a copy of the "import-helper" tests (with
a few modifications to allow a single annotation per line, as these
get rather long quickly!).
