API: Add a "submodule" param type

This allows action=paraminfo to indicate when a parameter is specifying
a submodule (so clients can detect what the submodule names are). This
will also allow the future action=help HTMLization to easily auto-link
to submodule docs, and will allow ApiSandbox to handle submodules more
generically.

For BC, rather than directly specifying "type": "submodule", it lists
the names of all the submodules as the type and adds an additional
"submodules" indicator on the param info object.

Change-Id: Id31babdc81d970ac781ec11daa3cdafef18ecd5d
Merge "API: Add a "submodule" param type"