Use full path for cross file links

[renjiexu]

Hi team,

In our company, we're using namespace such as following:

company
├── account
│   └── rpc
│       └── v1
│           └── account_api.proto
├── webhook
│   └── v1
│       └── webhook.proto
|       └── api.proto

instead of generating one html for all the *.proto, we generate one for each namespace:

company
├── account
│   └── rpc
│       └── v1
│           └── index.html
├── webhook
│   └── v1
│       └── index.html

However, if webhook.proto refers a type in account_api.proto, the link would not work.
This PR would address the problem, confirmed it's working for me, would like your thoughts on this before adding specs.

(might be able to solve the request of #406)

Thanks,
Renjie

[alkasm]

Bumping this, would be really interested here as well. We have way too many protos to comfortably fit on one page, so the per-folder or per-package approach with cross-linking would be perfect.

[renjiexu]

@alkasm good to know, I have a working solution for HTML/markdown in my forked version, will try to fix the unitest so we could have a clean PR to merge

[alkasm]

I think I found an issue with the generated links for nested message definitions. If you have

message Outer {
    message Inner { }
    Inner inner = 1;
}

Then protoc-gen-doc correctly makes the inner field link to #pkg.Outer.Inner, but this pr instead links to #/full/path/pkg.Outer.Inner. Either the initial # needs to be removed, or the full path needs to be removed when they're in the same file.