resolves #391

This pull request refactors the jitiRequire function in src/require.ts to resolve the issue with detecting prefix-only built-in modules, such as node:test.

I'd love to add tests, but I was unable to find any that deal with the node: protocol.

To test:

test.mts:

import { test } from 'node:test';

test('noop', {});

Run:

node --import jiti/register --test test.mts

Without this patch, a MODULE_NOT_FOUND error will occur.
With this patch, the script will successfully run.