feat: implement external command discovery for pixi extensions

[mrswastik-robot]

Implements external command discovery infrastructure following cargo's proven pattern, enabling pixi to find and recognize pixi-* executables in the system PATH.

Changes

• New module: src/cli/command_info.rs with complete external command discovery
• Core functions:
  • find_external_commands() - Discovers all pixi-* executables in PATH
  • find_external_subcommand() - Finds specific external command by name
  • search_directories() - PATH traversal following cargo's exact pattern
  • is_executable() - Cross-platform executable detection
  • builtin_exec() - Built-in command existence check

Next Steps

This lays the foundation for command dispatching logic, allowing us to integrate this discovery with the main CLI dispatcher.

Part of #3957
Parent issue - #3956

[nichmor]

This should serve as the foundation for handling external subcommands. The idea is that from this command::external place we can start finding and executing commands.

Hardcoding available commands is often not a good idea because it requires keeping track of and modifying them everywhere, which is prone to errors.

[nichmor]

I think it would be a good thing to have some integration tests ( in python ) that will test finding the pixi-extension and executing them correctly. You can take a look how we do it for pixi global

[nichmor]

One small change and by fixing the CI I think we can merge it

[ruben-arts]

Could you also test that it always takes the pixi commands first, e.g. pixi global install -e pixi-foobar --expose pixi-info=pixi-foobar

To make sure pixi itself always "wins"

[nichmor]

after a discussion with @ruben-arts - we may want in followup implementation did_you_mean error message

[nichmor]

also would be good to have some initial documentation ( pixi facing rather than pixi extension developer) around this behaviour

[Hofer-Julian]

Awesome work @mrswastik-robot and @nichmor 🚀

[nichmor]

looks good from my side