Use require for optional module imports

Webpack issues a warning when trying to import() a module that is
not present, whereas it does not do so with import(). Since we're
requiring a bundler anyway (since we're using npm package names
rather than import paths), we're not any worse off using a non-
standard module system for these.

For more info, see: webpack/webpack#7713

src/fetcher.ts

@@ -5,21 +5,17 @@ export const fetch: typeof window.fetch = (resource, init) => {
   // Implementation note: it's up to the client application to resolve these module names to the
   // respective npm packages. At least one commonly used tool (Webpack) is only able to do that if
   // the module names are literal strings.
-  const fetcherPromise = import(
-    // TypeScript cannot find the module - which is correct, since the consumer app should/can
-    // provide it:
-    // @ts-ignore
-    "@inrupt/solid-auth-fetcher"
-  )
-    .catch(() =>
-      import(
-        // TypeScript cannot find the module - which is correct, since the consumer app should/can
-        // provide it:
-        // @ts-ignore
-        "solid-auth-client"
-      )
-    )
-    .catch(() => import("cross-fetch"));
+  let fetch;
 
-  return fetcherPromise.then(({ fetch }) => fetch(resource, init));
+  try {
+    fetch = require("@inrupt/solid-auth-fetcher").fetch;
+  } catch (e) {
+    try {
+      fetch = require("solid-auth-client").fetch;
+    } catch (e) {
+      fetch = require("cross-fetch");
+    }
+  }
+
+  return fetch(resource, init);
 };