Fix fetching module scripts with circular imports

As pointed out by @jonco3 in IRC, the current fetching logic will
recurse infinitely if a module imports itself (directly or indirectly).
This fixes that by keeping track of all ancestors being fetched and not
re-fetching them.

Also, the "fetch a module script tree" and "fetch a single module
script" algorithms don't always complete with _new_ module scripts, if
they end up getting their result from the module map. So this removes
the word "new" in their preludes.

source

@@ -84318,10 +84318,13 @@ interface <dfn>NavigatorOnLine</dfn> {
   </ol>
 
   <p>To <dfn>fetch a module script tree</dfn> given a <var>url</var>, a <var>credentials mode</var>,
-  and a <var>settings object</var>, run these steps. The algorithm will asynchronously complete
-  with either null (on failure) or a new <span>module script</span> (on success).</p>
+  a <var>settings object</var>, and an optional <var>ancestor list</var>, run these steps. The
+  algorithm will asynchronously complete with either null (on failure) or a <span>module
+  script</span> (on success).</p>
 
   <ol>
+   <li><p>If <var>ancestor list</var> is not given, let it be an empty list.</p></li>
+
    <li><p><span>Fetch a single module script</span> given <var>url</var>, <var>credentials
    mode</var>, and <var>settings object</var>.</p>
 
@@ -84333,16 +84336,17 @@ interface <dfn>NavigatorOnLine</dfn> {
    these steps.</p></li>
 
    <li><p>Otherwise, <var>result</var> is a <span>module script</span>. <span data-x="fetch the
-   descendants of a module script">Fetch the descendants</span> of <var>result</var>.</p></li>
+   descendants of a module script">Fetch the descendants</span> of <var>result</var> using an
+   ancestor list obtained by appending <var>url</var> to <var>ancestor list</var>.</p></li>
 
    <li><p>When <span data-x="fetch the descendants of a module script">fetching the descendants of
    a module script</span> asynchronously completes with <var>descendants result</var>,
    asynchronously complete this algorithm with <var>descendants result</var>.</p></li>
   </ol>
 
-  <p>To <dfn>fetch the descendants of a module script</dfn> <var>module script</var>, run these
-  steps. The algorithm will asynchronously complete with either null (on failure) or with
-  <var>module script</var> (on success).</p>
+  <p>To <dfn>fetch the descendants of a module script</dfn> <var>module script</var>, using a given
+  <var>ancestor list</var>, run these steps. The algorithm will asynchronously complete with either
+  null (on failure) or with <var>module script</var> (on success).</p>
 
   <ol>
    <li><p>Let <var>record</var> be <var>module script</var>'s <span
@@ -84373,15 +84377,16 @@ interface <dfn>NavigatorOnLine</dfn> {
       </ol>
      </li>
 
-     <li><p>Otherwise, add <var>url</var> to <var>urls</var>.</p></li>
+     <li><p>Otherwise, if <var>url</var> is not in <var>ancestor list</var>, add <var>url</var> to
+     <var>urls</var>.</p></li>
     </ol>
    </li>
 
    <li>
     <p>For each <var>url</var> in <var>urls</var>, <span>fetch a module script tree</span> given
     <var>url</var>, <var>module script</var>'s <span
-    data-x="concept-module-script-credentials-mode">credentials mode</span>, and
-    <var>module script</var>'s <span>settings object</span>.</p>
+    data-x="concept-module-script-credentials-mode">credentials mode</span>, <var>module
+    script</var>'s <span>settings object</span>, and <var>ancestor list</var>.</p>
 
     <p>If any of the <span>fetch a module script tree</span> invocations asynchronously complete
     with null, the user agent may <span data-x="concept-fetch-terminate">terminate</span> any or
@@ -84395,7 +84400,7 @@ interface <dfn>NavigatorOnLine</dfn> {
 
   <p>To <dfn>fetch a single module script</dfn>, given a <var>url</var>, a <var>credentials
   mode</var>, and a <var>settings object</var>, run these steps. The algorithm will asynchronously
-  complete with either null (on failure) or a new <span>module script</span> (on success).</p>
+  complete with either null (on failure) or a <span>module script</span> (on success).</p>
 
   <ol>
    <li><p>Let <var>module map</var> be <var>settings</var>'s <span>module map</span>.</p></li>
@@ -116518,6 +116523,7 @@ INSERT INTERFACES HERE
   John Stockton,
   John-Mark Bell,
   Johnny Stenback,
+  Jon Coppeard,
   Jon Ferraiolo,
   Jon Gibbins,
   Jon Perlow,