Workaround for dub build-path problem

[bitwise]

https://github.com/dlang/dub/issues/658

As noted in the above issue, dub runs in the root project 
directory for all packages, including dependancies. So if any 
project aside from the root project includes a relative path in 
it's dub.json, that dub build will break, due to the incorrect 
working directory. I can't even use $PACKAGE_DIR to fix this 
because it doesn't work half the time (invalid identifier).

How do people deal with this?