feat: refactor spec implementation handling in global

[tdejager]

I've decided to split the task of installing source dependencies some more, this should lay a bit of the groundwork

Intro

This is the first step in a multi-phase implementation that will enable users to install packages from Git repositories, local
paths, and conda files directly - similar to cargo install --path ., npm install -g, and pipx install ..

Specifically, this PR focuses on refactoring the spec handling system to support the new GlobalSpec and PixiSpec formats required for the
upcoming source dependency features.

Before:

• pixi global install and pixi global add used MatchSpec handling
• No unified system for handling different dependency types

After:

• Introduced GlobalSpec and NamedGlobalSpec types for unified dependency handling
• Added GlobalSpecs CLI parser supporting --git, --path, --branch, --tag, --rev, and --subdir options
• Refactored manifest and project code to use the new spec system

Current limitation: Source dependencies are detected but filtered out with warning messages (full implementation coming in follow-up PRs)

Testing

Relying on global test suite for now, which is excellent (thanks @Hofer-Julian) helped me find some bugs and will extend it some more in subsequent PR's