I’m not following Pkg.jl development very closely but given that Julia 1.2 started supporting test/Project.toml I guess [extras] is going to fade away.
FYI, I also wanted to ease $subproject/Project.toml handling so I created a “task runner” package Run.jl (documentation). The API is that Run.script("DIRECTORY/SCRIPT.jl") automatically activate/instantiate DIRECTORY/Project.toml then run the script in an isolated environment. There are quick shortcuts Run.test() and Run.docs() but its useful for general “tasks”; e.g., running benchmarks in CI. All these sub-projects automatically Pkg.devs the parent directory so you can instantiate it even if your main project is not in the registry or does not have URL.