Allow subdirectories relative to @.

[simonbyrne]

It would be handy to be able to do julia --project=@./subdir and have it find the first parent Project.toml, then use subdir/Project.toml instead.

The use case is when running a project with optional GPU support: we have a main Project.toml file, then env/gpu/Project.toml with GPU support. Then from any subdirectory of the main repo I want to be able to julia --project=@./env/gpu and have it find the right file.

[StefanKarpinski]

Design question: should it be allowed to search beyond the root? Presumably since @. allows that but maybe not since in that case it would be useless otherwise.