Specifically, handle index and relative traversal expressions, and clean
up the code a little bit.
This should also help us pick up more `pulumi.interpolate` calls in TS.
Some property names are mapped from their `camelCase` Pulumi name to a
`snake_case` Python name. This mapping is irregular, and only occurs for
resources properties and function calls.
Note that there's still more work to do here: this only fixes names on
the output side; the input side is still broken for nested resource
proprerties and function calls.
The underlying design--annotated types in `hcl2/model`--may need some
additional work in the future, but I _believe_ it's good enough for now.
