How to setup dub project for contributing to a dub package?

[Ki Rill]

Recently, I tried to set up `dcv` with `dub` to improve a few 
things in the library, but I faced some strange issues.

When doing `dub add dcv`, it works fine. But when I try to add a 
local dcv project folder, or a repository, it fails to link the 
necessary package libraries. Seems like it's only `ffmpeg` 
libraries that are not linked.

What am I missing here?

```
// dub.json

...

"dependencies": {
     "dcv": {
         "repository": "git+https://github.com/rillki/dcv.git",
         "version": "~master"
     }
},

...
```

Output:
```
Undefined symbols for architecture x86_64:
   "_av_free_packet", referenced from:
       
__D3dcv7videoio5input11InputStream13readFrameImplMFNbNiKCQCc4core5image5ImageZb in libdcv.a(input_2c65_47c.o)
       
__D3dcv7videoio6output12OutputStream__T10writeSliceTS3mir7ndslice5slice__T9mir_sliceTPhVmi2VEQBoQBnQBi14mir_slice_kindi2ZQBvZQDiMFNbNiQDeEQFf4core5image11ImageFormatZb in libdcv.a(output_2c76_14e7.o)
       
__D3dcv7videoio6output12OutputStream__T10writeSliceTS3mir7ndslice5slice__T9mir_sliceTPhVmi3VEQBoQBnQBi14mir_slice_kindi2ZQBvZQDiMFNbNiQDeEQFf4core5image11ImageFormatZb in libdcv.a(output_2c74_14e8.o)
   "_av_register_all", referenced from:
       __D3dcv7videoio6common9AVStarter6__ctorMFNbNiZCQBsQBrQBmQBi 
in libdcv.a(common_2c56_37c.o)
   "_avcodec_decode_video2", referenced from:

       ETC....

ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to 
see invocation)
Error: linker exited with status 1
Error dmd failed with exit code 1.
```