`app add` should use the `sources` values from the app

[Keno]

Since each app has a standalone environment, it would be nice if it looked at the sources field of the app's Project.toml to find dependencies, e.g.:

keno@demeter3:~$ julia +nightly
A new `nightly` version is available. Install with `juliaup update`.
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.13.0-DEV.717 (2025-06-07)
 _/ |\__'_|_|_|\__'_|  |  Commit 51214a75240 (19 days old master)
|__/                   |

(@v1.13) pkg> app add https://github.com/Keno/ClaudeBox.jl
    Updating git-repo `https://github.com/Keno/ClaudeBox.jl`
ERROR: Unsatisfiable requirements detected for package BinaryBuilder2 [12aac903]:
 BinaryBuilder2 [12aac903] log:
 ├─BinaryBuilder2 [12aac903] has no known versions!
 └─restricted to versions 2 by ClaudeBox [a69d1bcb] — no versions left
   └─ClaudeBox [a69d1bcb] log:
     ├─possible versions are: 1.1.0 or uninstalled
     └─ClaudeBox [a69d1bcb] is fixed to version 1.1.0