Move dcd.common into separate subpackage

[WebFreak001]

This allows external projects to depend on dcd:common, which is needed to embed DCD clients.

Previously depending on DCD from dub would cause the whole server to be fetched with it, and with that also the dependencies libdparse, dsymbol, etc. which could cause conflicts with packages wanting to use DCD.

[WebFreak001]

@LaurentTreguier just requested a review from you in case you have time for this. Might be interesting for you if you plan to restart dls some time, as this is a quite simple change but changes where the build files are for common. If you had any tools that used DCD as submodule instead of with dub, they will probably break with this

[WebFreak001]

in my project I'm seeing considerably performance improvements on Windows when using DCD through the dcd.common interface, without breaking my dependencies.

Basically would just need some feedback if the code breakage, when using DCD as a git submodule, is ok.

There should be no change to packages using DCD as a dub dependency (those who depend on dcd-server). I have tried compiling DLS with this change, which uses DCD directly, and it seems to build fine.

[LaurentTreguier]

I haven't used DCD as a git submodule, and if I get back to DLS at some point, it might not even use the dlang-community tools; so this change is perfectly fine by me.
The breakage for git submodules means nothing more than a new minor release IMHO, just like other such changes