Allow displaying the derivation tree

[zanieb]

I need this for debugging error messages.

I used an environment variable instead of a trace log so you can do UV_INTERNAL__SHOW_DERIVATION_TREE=1 and run a test to see the tree in the test snapshot without further changes.

e.g.

    // Resolving should fail.
    uv_snapshot!(context.filters(), context.lock().arg("--preview").current_dir(&workspace), @r###"
    success: false
    exit_code: 1
    ----- stdout -----
    UV_INTERNAL__SHOW_DERIVATION_TREE
      root==0a0.dev0 depends on foo*
        root==0a0.dev0 depends on bar[some-extra]*
          foo==0.1.0 depends on anyio==4.1.0
            bar[some-extra]==0.1.0 depends on anyio==4.2.0
            no versions of bar[some-extra]<0.1.0 | >0.1.0

    ----- stderr -----
    Using Python 3.12.[X] interpreter at: [PYTHON-3.12]
      × No solution found when resolving dependencies:
      ╰─▶ Because only bar[some-extra]==0.1.0 is available and bar[some-extra] depends on anyio==4.2.0, we can conclude that all versions of bar[some-extra] depend on anyio==4.2.0.
          And because foo depends on anyio==4.1.0, we can conclude that foo and all versions of bar[some-extra] are incompatible.
          And because your workspace requires bar[some-extra] and foo, we can conclude that your workspace's requirements are unsatisfiable.
    "###
    );

[charliermarsh]

You can also use anstream::println.