using multiple DEPOT_PATH s with one julia installation

You might have `julialauncher` look in the default depot path as a fallback in case `DEPOT_PATH[1]` has no `juliaup` directory.

I'd like to use various main depots (the first entry in `DEPOT_PATH`) without running juliaup each time to install julia in the depot.

More precisely, I'd like to use juliaup to install to `~/.julia` and then later this same julia with various other depots. But, `~/.juliaup/bin/julia`, which links to the launcher of course can't find julia.

I am working on a project that manages a depot, etc. with scripts. If `~/.juliaup/bin/julia` is the first on my system path, then the real julia binary will not be found. I imagine that other scenarios like this might arise in the future.

Start by tracing julialauncher’s handling of DEPOT_PATH and the ~/.juliaup/bin/julia entry point. Reproduce the case with ~/.julia as a fallback depot and a different DEPOT_PATH[1]; done means the launcher finds the installed Julia without reinstalling it in each depot.

Written by the indexing model from the issue text.