Implemented something resembling offline mode.

This will address rust-lang#26 once we can merge this into master, until a
`cargo` with offline mode hits stable/crates.io this branch can be used
instead.

Cargo.toml

@@ -27,7 +27,7 @@ name = "rust-semver-public"
 path = "src/bin/rust_semver_public.rs"
 
 [dependencies]
-cargo = "0.36"
+cargo = { git = "https://github.com/rust-lang/cargo", branch = "master" }
 crates-io = "0.24"
 curl = "0.4.21"
 env_logger = "0.6"

src/bin/cargo_semver.rs

@@ -83,7 +83,7 @@ fn main() {
         &None, // color
         false, // frozen
         false, // locked
-        // matches.opt_present("offline"),
+        matches.opt_present("offline"),
         &None, // target_dir
         &[],   // unstable_flags
     );
@@ -441,7 +441,10 @@ impl<'a> WorkInfo<'a> {
 
             debug!("source id loaded: {:?}", source_id);
 
-            source.update()?;
+            if !config.offline() {
+                let _lock = config.acquire_package_cache_lock()?;
+                source.update()?;
+            }
 
             Box::new(source)
         };