cargo-lock: fix V3 lockfile handling and tests

The `version` field was previously being completely ignored.

This commit first fixes the tests so they run locally again (they were
broken in #533 but due to a quirk of how CI runs, namely nuking the
toplevel workspace Cargo.toml to support a 1.40 MSRV, they continued to
pass in CI).

It removes all previous dependencies on `cargo-lock` having a Cargo.lock
file (kind of silly to use something that changes as a test vector) and
replaces it with explicit V2 and V3 lockfile test vectors, which are now
in a newly renamed `tests/examples` directory.

Finally, it fixes parsing of V3 lockfiles, actually paying attention to
the `version` field if present.

cargo-lock/src/dependency/tree.rs

@@ -190,7 +190,7 @@ mod tests {
 
     /// Load this crate's `Cargo.lock`
     fn load_lockfile() -> Lockfile {
-        Lockfile::load("Cargo.lock").unwrap()
+        Lockfile::load("tests/examples/Cargo.lock.v3").unwrap()
     }
 
     #[test]

cargo-lock/src/lib.rs

@@ -21,7 +21,7 @@
 //! ```
 //! use cargo_lock::Lockfile;
 //!
-//! let lockfile = Lockfile::load("Cargo.lock").unwrap();
+//! let lockfile = Lockfile::load("tests/examples/Cargo.lock").unwrap();
 //! println!("number of dependencies: {}", lockfile.packages.len());
 //! ```
 //!

cargo-lock/src/lockfile/encoding.rs

@@ -74,7 +74,11 @@ impl TryFrom<EncodableLockfile> for Lockfile {
     type Error = Error;
 
     fn try_from(raw_lockfile: EncodableLockfile) -> Result<Lockfile, Error> {
-        let version = ResolveVersion::detect(&raw_lockfile.package, &raw_lockfile.metadata)?;
+        let version = match raw_lockfile.version {
+            Some(n) => n.try_into()?,
+            None => ResolveVersion::detect(&raw_lockfile.package, &raw_lockfile.metadata)?
+        };
+
         let mut packages = Vec::with_capacity(raw_lockfile.package.len());
 
         for raw_package in &raw_lockfile.package {

cargo-lock/src/lockfile/version.rs

@@ -58,16 +58,14 @@ impl ResolveVersion {
 
     /// Should this version be explicitly encoded?
     pub(super) fn is_explicit(self) -> bool {
-        u32::from(self) > 3
+        u32::from(self) >= 3
     }
 }
 
-/// V2 format is now the default.
-///
-///See: <https://github.com/rust-lang/cargo/pull/7579>
+/// V3 format is now the default.
 impl Default for ResolveVersion {
     fn default() -> Self {
-        ResolveVersion::V2
+        ResolveVersion::V3
     }
 }