feat: support out-of-tree builds

[lucascolley]

modulo #243, setting

[package.build.configuration]
env = { SRC_DIR = "./numpy" }

is a workaround to get out-of-tree builds working, when calling pixi build on that manifest.

However, if we add a source dependency on this package, the env var is no longer processed:

[dependencies]
numpy = { path = "./numpy" }

We see

 │ │ error: /Users/lucascolley/programming/pixi-dev-scipystack/numpy does not appear to be a Python project, as neither `pyproject.toml` nor `setup.py` are pres
 │ │ ent in the directory

regardless of the value of package.build.configuration.env.SRC_DIR.

cc @baszalmstra