Clean up FileType enum following enum namespacing

All of the enum components had a redundant 'Type' specifier: TypeSymlink, TypeDirectory, TypeFile. This change removes them, replacing them with a namespace: FileType::Symlink, FileType::Directory, and FileType::RegularFile.

RegularFile is used instead of just File, as File by itself could be mistakenly thought of as referring to the struct.

[breaking-change]

src/test/run-pass/issue-18619.rs

@@ -11,5 +11,5 @@
 use std::io::FileType;
 
 pub fn main() {
-    let _ = FileType::TypeFile.clone();
+    let _ = FileType::RegularFile.clone();
 }