Auto merge of #11630 - Muscraft:fix-unused-manifest-keys, r=ehuss

fix(toml): Provide a way to show unused manifest keys for dependencies

Dependencies have not been able to show unused manifest keys for some time, this problem partially resulted in #11329.

This problem is caused by having an `enum` when deserializing.  To get around this you can use:
```rust
#[serde(flatten)]
other: BTreeMap<String, toml::Value>,
```
This collects any unused keys into `other` that can later be used to show warnings. This idea was suggested in a thread I cannot find but is mentioned in [serde#941](serde-rs/serde#941).

Author: bors

src/cargo/core/dependency.rs

@@ -106,6 +106,16 @@ pub enum DepKind {
     Build,
 }
 
+impl DepKind {
+    pub fn kind_table(&self) -> &'static str {
+        match self {
+            DepKind::Normal => "dependencies",
+            DepKind::Development => "dev-dependencies",
+            DepKind::Build => "build-dependencies",
+        }
+    }
+}
+
 impl ser::Serialize for DepKind {
     fn serialize<S>(&self, s: S) -> Result<S::Ok, S::Error>
     where