Fix relative paths (#4594)

* Add tests for relativeRoot

* Remove path.posix.join

Since this is for file system paths it feels incorrect to use it on
URL paths as they are different in many ways.

* Rewrite cookie path logic

Before we relied on the client to resolve the base given to it by the
backend against the path.

Instead have the client pass that information along so we can resolve it
on the backend.  This means the client has to do less work.

* Do not remove out directory before watch

This is re-used for incremental compilation.

Also remove del since that was the only use (and we can use fs.rmdir in
the future if we need something like this).

* Remove unused function resolveBase

src/browser/pages/login.html

@@ -30,7 +30,8 @@
         <div class="content">
           <form class="login-form" method="post">
             <input class="user" type="text" autocomplete="username" />
-            <input id="base" type="hidden" name="base" value="/" />
+            <input id="base" type="hidden" name="base" value="{{BASE}}" />
+            <input id="href" type="hidden" name="href" value="" />
             <div class="field">
               <input
                 required
@@ -51,9 +52,9 @@
     <script>
       // Inform the backend about the path since the proxy might have rewritten
       // it out of the headers and cookies must be set with absolute paths.
-      const el = document.getElementById("base")
+      const el = document.getElementById("href")
       if (el) {
-        el.value = window.location.pathname
+        el.value = location.href
       }
     </script>
   </body>