feat(cargo-lock): support lockfile v4

In 1.78.0 Cargo introduce v4 lockfile:

rust-lang/cargo#12852

The v3-to-v4 change is minimal: encode URL params with URL encoding.
However, roundtrip test failed because v4 we can only have one
serialization implementation at a time.

I don't know how to proceed. Maybe we could

* Make v4 serialization the default when it becomes the default in Cargo
* Provide API for people to generate different versions of lockfiles.
* Work with upstream Cargo with an in-tree pacakge for lock serialization.
  (This is on me?)

Opened this PR for discussion.

cargo-lock/tests/lockfile.rs

@@ -15,6 +15,9 @@ const V2_LOCKFILE_PATH: &str = "tests/examples/Cargo.lock.v2";
 /// Path to a V3 `Cargo.lock` file.
 const V3_LOCKFILE_PATH: &str = "tests/examples/Cargo.lock.v3";
 
+/// Path to a V4 `Cargo.lock` file.
+const V4_LOCKFILE_PATH: &str = "tests/examples/Cargo.lock.v4";
+
 /// Load example V1 `Cargo.lock` file (from the Cargo project itself)
 #[test]
 fn load_example_v1_lockfile() {
@@ -68,6 +71,13 @@ fn serialize_v3() {
     assert_eq!(lockfile2.packages, lockfile.packages);
 }
 
+/// Load example V4 `Cargo.lock` file
+#[test]
+fn load_example_v4_lockfile() {
+    let err = Lockfile::load(V4_LOCKFILE_PATH).unwrap_err();
+    assert_eq!(&err.to_string(), "parse error: parse error: invalid Cargo.lock format version: `4`\n");
+}
+
 /// Ensure we can serialize a V2 lockfile as a V1 lockfile
 #[test]
 fn serialize_v2_to_v1() {