feat: add the package feature unification option

Author: mladedav

src/cargo/ops/resolve.rs

@@ -159,6 +159,7 @@ pub fn resolve_ws_with_opts<'gctx>(
         FeatureUnification::Workspace => {
             vec![ops::Packages::All(Vec::new()).to_package_id_specs(ws)?]
         }
+        FeatureUnification::Package => specs.iter().map(|spec| vec![spec.clone()]).collect(),
     };
     let specs: Vec<_> = individual_specs
         .iter()

src/cargo/util/context/mod.rs

@@ -2863,6 +2863,7 @@ pub enum IncompatibleRustVersions {
 #[derive(Copy, Clone, Debug, Deserialize)]
 #[serde(rename_all = "kebab-case")]
 pub enum FeatureUnification {
+    Package,
     Selected,
     Workspace,
 }