Fix injected errors when running doctests on a crate named after a keyword

Unfortunately, this can't currently be tested. The problem is that we
need the file to be compiled first to then be used as dependency, which
cannot be done currently unfortunately in the rustdoc test suites.
Example:

```rust
// name this file "foo.rs"

/// ```
/// let x = foo::foo();
/// ```
pub fn foo() {}
```

If you run `rustdoc --test foo.rs`, you'll get:

```
running 1 test
test foo.rs - foo (line 1) ... FAILED

failures:

---- foo.rs - foo (line 1) stdout ----
error[E0463]: can't find crate for `foo`
 --> foo.rs:0:1
  |
2 | extern crate foo;
  | ^^^^^^^^^^^^^^^^^ can't find crate
```

If a test were possible, it would look something like

 ````rust
 #![crate_name = "mod"]
 #![crate_type = "lib"]
 //! ```
 //! // NOTE: requires that the literal string 'mod' appears in the doctest for
 //! // the bug to appear
 //! assert_eq!(1, 1);
 //! ```
 ````

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