WIP: Use pkg for external deps

[vesavlad]

The idea behind this PR is to use PKG tool https://github.com/motis-project/pkg in order to manage external dependencies with keeping a lightweight project structure.

This is a WIP since it relies on some changes that have to happen on the linked (from now on) submodules.

[vesavlad]

@patrickbr what do you think about this? the idea is to make this a step into merging so more changes related to PBF processing using a osmium version that does not need BOOST as a dependency to work

But as you can see some changes need to happen to the submodules in order for them to be used a proper c++ cmake libraries

[patrickbr]

Thanks, interesting idea. Glancing over the example in the pkg README, I don't quite understand the difference to vanilla git submodules. The example file looks very much like a .gitmodules file. Could you explain in a sentence or two what the difference is?