From the linked page:
A few general rules to note:
…
2. A package with a project file cannot depend on one without a project file since packages with project files can only load packages in graph and packages without project files do not appear in graph.
I am waiting for the upcoming workspace feature in Pkg.jl, which will hopefully simplify the organization of such mono repos.
This discussion here might also be of interest to you: Create an implicit environment with package directories