uv tree: include workspace members in roots

[j178]

Summary

For a workspace, when one member depends on another, uv tree and uv tree --package does not show the denpendent member.

For example:

$ uv init project
$ cd project
$ uv init foo
$ uv init bar
$ uv add --package foo bar
$ uv tree
Resolved 3 packages in 4ms
foo v0.1.0
└── bar v0.1.0
project v0.1.0
$ uv tree --package bar
Resolved 3 packages in 5ms

This PR makes sure workspace members are always included in the roots, even if they are dependencies, so uv tree will display them as roots.

After this PR:

$ cargo run -- tree
Resolved 3 packages in 11ms
bar v0.1.0
foo v0.1.0
└── bar v0.1.0
project v0.1.0
$ cargo run -- tree --package bar
Resolved 3 packages in 9ms
bar v0.1.0

[charliermarsh]

Should we just get rid of non_roots entirely then, and rely solely on the list of members?