Fix check for existing crate when using --extern

When checking for an existing crate, compare against the
`crate_metadata::name` field, which is the crate name which
was requested during resolution, rather than the result of the
`crate_metadata::name()` method, which is the crate name within
the crate metadata, as these may not match when using the --extern
option to `rustc`.

This fixes spurious "multiple crate version" warnings under the
following scenario:

- The crate `foo`, is referenced multiple times
- `--extern foo=./path/to/libbar.rlib` is specified to rustc
- The internal crate name of `libbar.rlib` is not `foo`

The behavior surrounding `Context::should_match_name` and the
comments in `loader.rs` both lead me to believe that this scenario
is intended to work.

Fixes #17186

src/librustc/metadata/creader.rs

@@ -281,7 +281,7 @@ fn existing_match(e: &Env, name: &str,
                   hash: Option<&Svh>) -> Option<ast::CrateNum> {
     let mut ret = None;
     e.sess.cstore.iter_crate_data(|cnum, data| {
-        if data.name().as_slice() != name { return }
+        if data.name.as_slice() != name { return }
 
         match hash {
             Some(hash) if *hash == data.hash() => { ret = Some(cnum); return }