Factor buildName into reusable function

The new function is chisel3.internal.buildName.

core/src/main/scala/chisel3/RawModule.scala

@@ -154,6 +154,20 @@ package object internal {
   /** Marker trait for modules that are not true modules */
   private[chisel3] trait PseudoModule extends BaseModule
 
+  /** Creates a name String from a prefix and a seed
+    * @param prefix The prefix associated with the seed (must be in correct order, *not* reversed)
+    * @param seed The seed for computing the name (if available)
+    */
+  def buildName(seed: String, prefix: Prefix): String = {
+    val builder = new StringBuilder()
+    prefix.foreach { p =>
+      builder ++= p
+      builder += '_'
+    }
+    builder ++= seed
+    builder.toString
+  }
+
   // Private reflective version of "val io" to maintain Chisel.Module semantics without having
   // io as a virtual method. See https://github.com/freechipsproject/chisel3/pull/1550 for more
   // information about the removal of "val io"

core/src/main/scala/chisel3/internal/Builder.scala

@@ -171,22 +171,6 @@ private[chisel3] trait HasId extends InstanceId {
     * @return the name, if it can be computed
     */
   private[chisel3] def _computeName(defaultPrefix: Option[String], defaultSeed: Option[String]): Option[String] = {
-
-    /** Computes a name of this signal, given the seed and prefix
-      * @param seed
-      * @param prefix
-      * @return
-      */
-    def buildName(seed: String, prefix: Prefix): String = {
-      val builder = new StringBuilder()
-      prefix.foreach { p =>
-        builder ++= p
-        builder += '_'
-      }
-      builder ++= seed
-      builder.toString
-    }
-
     if (hasSeed) {
       Some(buildName(seedOpt.get, naming_prefix.reverse))
     } else {