How to compile against GitHub fork of Phobos?

[Q. Schroll via Digitalmars-d-learn]

I have a fork of the standard-library in the folder "phobos". In 
version 2.072.1 of the compiler, I could use code like

     void main()
     {
         import phobos.std.format;
         /* code for checking my changes to format */
     }

compiled with

   $ dmd -I"phobos" test_format.d

in the parent folder of phobos. It worked fine.

I've updated the compiler today and get an error message:

     module std.typecons from file phobos\std\typecons.d must be 
imported with 'import std.typecons;'

But if I do so, it imports the one from the standard library 
attached to the compiler. Is it a regression? Should the old 
version have rejected my code? I don't see any possibility to 
test specific changes in modules of my fork.

I've just read 
https://wiki.dlang.org/Starting_as_a_Contributor#Building_D, but 
it didn't help me out.