Add GoFile and move HtmlFile to Files.qll

ql/src/semmle/go/Files.qll

@@ -264,3 +264,13 @@ class File extends ExtractedOrExternalFile {
     exists(this.getAChild())
   }
 }
+
+/** A Go file. */
+class GoFile extends File {
+  GoFile() { this.getExtension() = "go" }
+}
+
+/** An HTML file. */
+class HtmlFile extends File {
+  HtmlFile() { this.getExtension().regexpMatch("x?html?") }
+}

ql/src/semmle/go/HTML.qll

@@ -3,13 +3,6 @@
 import go
 
 module HTML {
-  /**
-   * An HTML file.
-   */
-  class HtmlFile extends File {
-    HtmlFile() { this.getExtension().regexpMatch("x?html?") }
-  }
-
   /**
    * An HTML element.
    *