Explicitly select members in a workspace

If there is a workspace with multiple members (crate1, crate2) which use the same crate dependency (crate3) but crate1 uses a feature feature_a and crate2 doesn't, then rust-analyzer won't Identify code under #[cfg(not(feature = "feature_a"))], even though crate2 might use such code.

In order to solve this straightforwardly, it would be nice to have the option to either

1. Make rust-analyzer discover only members in default-members.
2. Define explicit members for rust-analyzer to discover, similar to the rust-analyzer.linkedProjects setting.

Example Cargo.toml:

[workspace]
resolver = "2"
members = [
    "crate1",
    "crate2",
]

default-members = [
    "crate2"
]

[workspace.dependencies]
crate3 = { path = "crate3", default-features = false }

Start by reproducing the workspace shown in the issue and inspect rust-analyzer's workspace-member discovery and Cargo configuration handling. The work is complete when rust-analyzer can explicitly select discovered members, including the proposed default-members behavior, and cfg analysis reflects the selected members.