Fix --extern library finding errors

- `crate_name` is not specified/passed to `metadata_crate_location_unknown_type`
https://github.com/rust-lang/rust/blob/c493bae0d8efd75723460ce5c371f726efa93f15/compiler/rustc_error_messages/locales/en-US/metadata.ftl#L274-L275
- `metadata_lib_filename_form` is missing `$`
- Add additional check to ensure that library is file

Testing
1. Create file `a.rs`
```rust
extern crate t;
fn main() {}
```
1. Create empty file `x`
1. Create empty directory `y`
1. Run
```sh
$ rustc -o a a.rs --extern t=x
$ rustc -o a a.rs --extern t=y
```
Both currently panic with stable.

…earth

Rollup of 6 pull requests

Successful merges:

 - rust-lang#103488 (Allow opaque types in trait impl headers and rely on coherence to reject unsound cases)
 - rust-lang#104359 (Refactor must_use lint into two parts)
 - rust-lang#104612 (Lower return type outside async block creation)
 - rust-lang#104621 (Fix --extern library finding errors)
 - rust-lang#104647 (enable fuzzy_provenance_casts lint in liballoc and libstd)
 - rust-lang#104750 (Bump `fd-lock` in `bootstrap` again)

Failed merges:

 - rust-lang#104732 (Refactor `ty::ClosureKind` related stuff)

r? `@ghost`
`@rustbot` modify labels: rollup