Adjust .stderr files

tests/ui/external_subcommand_wrong_type.stderr

@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/external_subcommand_wrong_type.rs:13:15
    |
 13 |     Other(Vec<CString>)
-   |               ^^^^^^^ expected struct `std::ffi::CString`, found struct `std::ffi::OsString`
+   |               ^^^^^^^ expected struct `CString`, found struct `OsString`
    |
-   = note: expected struct `std::vec::Vec<std::ffi::CString>`
-              found struct `std::vec::Vec<std::ffi::OsString>`
+   = note: expected struct `Vec<CString>`
+              found struct `Vec<OsString>`

tests/ui/non_existent_attr.stderr

@@ -1,5 +1,5 @@
-error[E0599]: no method named `non_existing_attribute` found for struct `structopt::clap::Arg<'_, '_>` in the current scope
+error[E0599]: no method named `non_existing_attribute` found for struct `Arg<'_, '_>` in the current scope
   --> $DIR/non_existent_attr.rs:14:24
    |
 14 |     #[structopt(short, non_existing_attribute = 1)]
-   |                        ^^^^^^^^^^^^^^^^^^^^^^ method not found in `structopt::clap::Arg<'_, '_>`
+   |                        ^^^^^^^^^^^^^^^^^^^^^^ method not found in `Arg<'_, '_>`

tests/ui/skip_without_default.stderr

@@ -1,9 +1,7 @@
-error[E0277]: the trait bound `Kind: std::default::Default` is not satisfied
+error[E0277]: the trait bound `Kind: Default` is not satisfied
   --> $DIR/skip_without_default.rs:22:17
    |
 22 |     #[structopt(skip)]
-   |                 ^^^^ the trait `std::default::Default` is not implemented for `Kind`
+   |                 ^^^^ the trait `Default` is not implemented for `Kind`
    |
    = note: required by `std::default::Default::default`
-
-For more information about this error, try `rustc --explain E0277`.