Support Module name overrides with "override def desiredName"

The API allowed this before, but not safely, as users could create
name conflicts.  This exposes the pre-deduplication/sanitization
naming API, and closes the other one.

chiselFrontend/src/main/scala/chisel3/core/Module.scala

@@ -57,8 +57,11 @@ extends HasId {
   private[core] val _ids = ArrayBuffer[HasId]()
   dynamicContext.currentModule = Some(this)
 
-  /** Name of the instance. */
-  val name = Builder.globalNamespace.name(getClass.getName.split('.').last)
+  /** Desired name of this module. */
+  def desiredName = this.getClass.getName.split('.').last
+
+  /** Legalized name of this module. */
+  final val name = Builder.globalNamespace.name(desiredName)
 
   /** IO for this Module. At the Scala level (pre-FIRRTL transformations),
     * connections in and out of a Module may only go through `io` elements.

src/main/scala/chisel3/internal/firrtl/Emitter.scala

@@ -46,18 +46,21 @@ private class Emitter(circuit: Circuit) {
   }
 
   // Map of Module FIRRTL definition to FIRRTL name, if it has been emitted already.
-  private val defnMap = collection.mutable.HashMap[String, String]()
+  private val defnMap = collection.mutable.HashMap[(String, String), Component]()
   // Map of Component name to FIRRTL id.
   private val moduleMap = collection.mutable.HashMap[String, String]()
 
-  /** Generates the FIRRTL module definition with a specified name.
+  /** Generates the FIRRTL module declaration.
     */
-  private def moduleDefn(m: Component, name: String): String = {
+  private def moduleDecl(m: Component): String = m.id match {
+    case _: BlackBox => newline + s"extmodule ${m.name} : "
+    case _: Module => newline + s"module ${m.name} : "
+  }
+
+  /** Generates the FIRRTL module definition.
+    */
+  private def moduleDefn(m: Component): String = {
     val body = new StringBuilder
-    m.id match {
-      case _: BlackBox => body ++= newline + s"extmodule $name : "
-      case _: Module => body ++= newline + s"module $name : "
-    }
     withIndent {
       for (p <- m.ports)
         body ++= newline + emitPort(p)
@@ -82,21 +85,20 @@ private class Emitter(circuit: Circuit) {
     */
   private def emit(m: Component): String = {
     // Generate the body.
-    val moduleName = m.id.getClass.getName.split('.').last
-    val defn = moduleDefn(m, moduleName)
+    val defn = moduleDefn(m)
 
-    defnMap get defn match {
-      case Some(deduplicatedName) =>
-        moduleMap(m.name) = deduplicatedName
+    defnMap get (m.id.desiredName, defn) match {
+      case Some(duplicate) =>
+        moduleMap(m.name) = duplicate.name
         ""
       case None =>
         require(!(moduleMap contains m.name),
             "emitting module with same name but different contents")
 
         moduleMap(m.name) = m.name
-        defnMap(defn) = m.name
+        defnMap((m.id.desiredName, defn)) = m
 
-        moduleDefn(m, m.name)
+        moduleDecl(m) + defn
     }
   }