Java2SwiftPlugin: Drop the ability to infer Swift type names

The configuration file should be explicit. If we want to infer Swift type
names, we should do it when generating the configuration file.

Author: DougGregor

Plugins/Java2SwiftPlugin/Configuration.swift

@@ -20,7 +20,6 @@ struct Configuration: Codable {
 
   /// The Java classes that should be translated to Swift. The keys are
   /// canonical Java class names (e.g., java.util.Vector) and the values are
-  /// the corresponding Swift names (e.g., JavaVector). If the value is `nil`,
-  /// then the Java class name will be used for the Swift name, too.
-  var classes: [String: String?] = [:]
+  /// the corresponding Swift names (e.g., JavaVector).
+  var classes: [String: String] = [:]
 }

Plugins/Java2SwiftPlugin/Java2SwiftPlugin.swift

@@ -75,9 +75,7 @@ struct Java2SwiftBuildToolPlugin: BuildToolPlugin {
     /// Determine the list of Java classes that will be translated into Swift,
     /// along with the names of the corresponding Swift types. This will be
     /// passed along to the Java2Swift tool.
-    let classes = config.classes.map { (javaClassName, swiftName) in
-      (javaClassName, swiftName ?? javaClassName.defaultSwiftNameForJavaClass)
-    }.sorted { (lhs, rhs) in
+    let classes = config.classes.sorted { (lhs, rhs) in
       lhs.0 < rhs.0
     }
 
@@ -139,16 +137,3 @@ func findJavaHome() -> String {
 
   fatalError("Please set the JAVA_HOME environment variable to point to where Java is installed.")
 }
-
-extension String {
-  /// For a String that's of the form java.util.Vector, return the "Vector"
-  /// part.
-  fileprivate var defaultSwiftNameForJavaClass: String {
-    if let dotLoc = lastIndex(of: ".") {
-      let afterDot = index(after: dotLoc)
-      return String(self[afterDot...])
-    }
-
-    return self
-  }
-}