feat(godocfx): add two levels of nesting to TOC

[tbpg]

The TOC is a bit unruly with every package in a single list. So, I added a couple levels of nesting: one for the module and one for the first part after the module. In other words, I split package names into mod/mid/suffix and nest shared mod and mid values. If there would only be one element in the nested list, it isn't nested. This is like a trie with only two levels.

The result looks like this:

- uid: cloud.google.com/go/bigquery
  name: cloud.google.com/go/bigquery
  items:
  - name: README
    href: README.md
  - name: connection
    items:
    - uid: cloud.google.com/go/bigquery/connection/apiv1
      name: apiv1
    - uid: cloud.google.com/go/bigquery/connection/apiv1beta1
      name: apiv1beta1
  - uid: cloud.google.com/go/bigquery/datatransfer/apiv1
    name: datatransfer/apiv1

[codyoss]

Good idea! LGTM