Widen understanding of prelude import

Prelude imports are exempt from wildcard import warnings. Until now only
imports of the form

```
use ...::prelude::*;
```

were considered. This change makes it so that the segment `prelude` can
show up anywhere, for instance:

```
use ...::prelude::v1::*;
```

Fixes rust-lang#5917

Author: stchris

clippy_lints/src/wildcard_imports.rs

@@ -195,13 +195,10 @@ impl WildcardImports {
     }
 }
 
-// Allow "...prelude::*" imports.
+// Allow "...prelude::..::*" imports.
 // Many crates have a prelude, and it is imported as a glob by design.
 fn is_prelude_import(segments: &[PathSegment<'_>]) -> bool {
-    segments
-        .iter()
-        .last()
-        .map_or(false, |ps| ps.ident.as_str() == "prelude")
+    segments.iter().filter(|ps| ps.ident.as_str() == "prelude").count() > 0
 }
 
 // Allow "super::*" imports in tests.

tests/ui/auxiliary/wildcard_imports_helper.rs

@@ -19,3 +19,9 @@ mod extern_exports {
         A,
     }
 }
+
+pub mod prelude {
+    pub mod v1 {
+        pub struct PreludeModAnywhere;
+    }
+}

tests/ui/wildcard_imports.rs

@@ -20,6 +20,7 @@ use wildcard_imports_helper::inner::inner_for_self_import::*;
 use wildcard_imports_helper::*;
 
 use std::io::prelude::*;
+use wildcard_imports_helper::prelude::v1::*;
 
 struct ReadFoo;
 
@@ -75,6 +76,7 @@ fn main() {
     let _ = A;
     let _ = inner_struct_mod::C;
     let _ = ExternA;
+    let _ = PreludeModAnywhere;
 
     double_struct_import_test!();
     double_struct_import_test!();