feat(pixi_spec): support extras, flags and when on conda dependencies

[Hofer-Julian]

This enables extras, flags and when to work with source dependencies.
Also, recursive source dependencies work.

Example 1:

[dependencies]
example-rust = { path = ".", extras = ["wolf", "julian"]}

[package]
name = "example-rust"
version = "0.1.0"

[package.build]
backend = { name = "pixi-build-rust", version = "*"  }

[package.extra-dependencies.wolf]
rust = "*"
example-rattler-build = { path = "./recipe/recipe.yaml" }

[package.target.win-64.extra-dependencies.julian]
python = "*"

Example 2:

[dependencies]
example-rust = { path = ".", flags = ["cuda"] }

[package]
name = "example-rust"
version = "0.1.0"

[package.build]
backend = { name = "pixi-build-rust", version = "*"  }
flags = ["cuda", "blas_openblas"]

Example 3:

[dependencies]
example-rust = { path = "." }
python = "3.13"

[package]
name = "example-rust"
version = "0.1.0"

[package.build]
backend = { name = "pixi-build-rust", version = "*"  }

[package.run-dependencies]
numpy = { version = "*", when = { package = "python", version = ">=3.12" } }

[baszalmstra]

This PR merges some fundamental concepts.

Previously we had SourceLocationSpec which is just a reference to a source location. We renamed that to SourceSpec and added matchspec fields to the type. However, we also want to be able to refer to just the location, for instance when refering to where the manifest is or where the build source is. This now also always carries a matchspec which makes no sense in this context. I think we should split these concepts, have a SourceLocationSpec which refers to a location that we can checkout and where source is located, and a combination of a SourceLocationSpec and matchspec fields to refer to a specific package variant inside the source location.