Use Promise.all to fetch files part of py-config (#1322)

This is a first step towards loading more stuff simultaneously rather
than sequentially.

The functional part of this is pretty small: call `calculateFetchPaths` and
then `Promise.all(fetchPaths.map(loadFileFromURL));`. I also transposed the
return type of `calculateFetchPaths` since it's more convenient to consume
this way.

I redid the logic in `calculateFetchPaths` a bit. I renamed `src/plugins/fetch.ts`
to `calculateFetchPaths.ts` since the file performs no fetching. I also
renamed `loadFromFile` to `loadFileFromURL`.

pyscriptjs/src/utils.ts

@@ -80,7 +80,7 @@ export function joinPaths(parts: string[], separator = '/') {
         .map(function (part) {
             return part.trim().replace(/(^[/]*|[/]*$)/g, '');
         })
-        .filter(p => p !== '')
+        .filter(p => p !== '' && p !== '.')
         .join(separator || '/');
     if (parts[0].startsWith('/')) {
         return '/' + res;