Suppose that I fork a package and do some modifications to it's code. Now, I'd like to use this modified version instead of the original one shipped with Pkg. How can I do this?
So it's worth noting that Packages don't ship with Pkg itself. The Package manager is connected with the Julia General Registry (hosted on GitHub) in most cases but you can connect it with another public or private package registry as well. You can also remove the connection to the registry but still have the Package Manager installed and working.
If you want to just use a forked version that you are not locally developing, you can do something like:
(v1.0) pkg> add https://github.com/fredrikekre/ImportMacros.jl
Note to enter the Pkg mode from the Repl, you have to type ].
If you are actively developing the code from the fork locally, you should follow the example here: https://julialang.github.io/Pkg.jl/v1/managing-packages/#developing
Find out more here: https://julialang.github.io/Pkg.jl/v1/managing-packages/#**3.**-Managing-Packages