Provide a way to describe why newer versions are not selected

[ehuss]

Describe the problem you are trying to solve
Sometimes it can be confusing why newer versions of a package are not selected by the resolver. It would be nice to have some way for Cargo to tell you that there is a newer version available, but it is not able to select it due to some requirement.

There are different reasons this can happen. Some I can think of:

• Multiple version constraints of different styles. For example one package has "1.0.*", and another has "^1.0".
• Features were removed in an otherwise semver-compatible release. Packages probably shouldn't do that, but it can still be silently confusing.

This is in spirit with #6199 and #5284, but this is not for errors but for when otherwise compatible versions are skipped.

Describe the solution you'd like
I'm not sure of the best way to convey this information. Maybe cargo update -v could include a detailed message? Or maybe make it part of CARGO_LOG? Or something else?

It could say something like:

info: a newer version of package `bar` is available, 
but it is unable to be selected due to restricted requirements
  `bar` 1.1.0 is available, but
    `somedep`'s requirement `^1.0` selected older version `1.0.0` because
    `restrictor` has the requirement `1.0.*` which prevents the newer version from being selected    

Example
Some examples using Cargo's test notation:

#[cargo_test]
fn describe_restricted_dep() {
    // Example showing a newer version not selected due to an unrelated
    // package's over-restrictive requirements.
    Package::new("dep", "1.0.0").publish();
    Package::new("dep", "1.1.0").publish();
    Package::new("restrictor", "1.0.0")
        .dep("dep", "1.0.*")
        .publish();

    let p = project()
        .file(
            "Cargo.toml",
            r#"
            [package]
            name = "foo"
            version = "1.0.0"

            [dependencies]
            restrictor = "1.0"
            dep = "1.0"
            "#,
        )
        .file("src/lib.rs", "")
        .build();

    p.cargo("update -v").with_stderr("[UPDATING] [..]").run();
}

#[cargo_test]
fn describe_misfeatured_dep() {
    // Example showing a newer version not selected due to missing features.
    Package::new("dep", "1.0.0").feature("f1", &[]).publish();
    Package::new("dep", "1.0.1").publish();

    let p = project()
        .file(
            "Cargo.toml",
            r#"
            [package]
            name = "foo"
            version = "1.0.0"

            [dependencies]
            dep = { version = "1.0", features = ["f1"] }
            "#,
        )
        .file("src/lib.rs", "")
        .build();

    p.cargo("update -v").with_stderr("[UPDATING] [..]").run();
}

[epage]

#6666 also asked for this (among other things) but it was closed for #12903 which at least tells people how to enable logging to get the details they want. If we can directly tell them, that'd be great. I worry anything we try will be a lot of work and will be information overload, making it not much better than the logging solution.