Use `System.getImportMap()` instead of an additional fetch

[romulof]

SystemJS 6.15 introduced a new API to expose its import map:

https://github.com/systemjs/systemjs/pull/2484

This could be conditionally enabled in a implementation like bellow, so it wouldn't force people to upgrade to 6.15:

if (typeof System.getImportMap === 'function') {
   return System.getImportMap();
} else {
   return fetch(...);
}