pbjs/sparse: fix namespace replacement, include transitive dependencies

[imirkin]

This fixes sparse mode for my use-case. Protos are spread over several files, and may include nested messages/enums from other files. Additionally, this fixes the case where a Type is replaced by a Namespace, e.g. when an enum is used without its containing message type.

Fixes #1436

[alexander-fenster]

This looks reasonable, we'll just need some more time to review the change and we'll get back to you @imirkin. Thanks for the PR!

[binsee]

Is there any progress on this please? @alexander-fenster

[imirkin]

Is there any chance this will ever get in? Should I just close it? Rebase it?

I can't use --sparse without this patch, since the current logic ends up over-removing types which are actually in use.