Adding Github repo does not add [sources] section in Project.toml

[mitiemann]

Adding a Github repo does not add an [sources] entry in the Project.toml, leading to an unhelpful error message. This happens both in Pkg REPL mode and in API mode.

MWE:

mkdir {A,B}
cd A
julia -e 'import Pkg; Pkg.activate("."); Pkg.add(url="https://github.com/tlienart/Xranklin.jl.git")'
cd ../B
cp ../A/Project.toml .
julia --project=@. -e 'import Pkg; Pkg.instantiate()'

throws a pkgerror:

ERROR: expected package `Xranklin [558449b0]` to be registered

[jakobnissen]

Discussed in #3826 and #3783 (comment)

[mitiemann]

Thanks! Is there anything that I could help with to move #3826 forwards?