Omit workspace members for --no-sources commands

[charliermarsh]

Summary

In uv sync and friends, even with --no-sources, we discover all the workspace members and send them in as local editable dependencies before we even hit lowering. This PR filters them out during discovery.

Honestly, I'm not sure if we want this behavior. It means that if you have a member that depends on a workspace member, and that dependency isn't available through another source (e.g., from PyPI), resolution will fail with --no-sources, since we'll ignore workspace = true. But if we don't have this behavior, then we can't plausibly support replacing workspace dependencies with other dependencies via --no-sources as described in #18258. Semantically, though, ignoring workspace = true does seem correct, and --no-sources-package exists for more targeted usages.

Closes #18258.

[zanieb]

Hm while this is technically more consistent I'm pretty worried it's not actually desirable. I'll try to think of an alternative.

[charliermarsh]

I think the thing we need to figure out is: if we always enforce that workspace dependencies are resolved to workspace members, is there a supported way to patch or override them? I'm concerned that we've probably recommended this exact approach in the past to solve that problem.

[zanieb]

Separately from the existing behavior, I think I might expect --no-workspace to do that.

[charliermarsh]

(To be clear, I'm not sold on this change either -- oh, I guess I said this in the summary. I mostly care that the behavior is consistent between commands...)