require method on GeneratorContext should fall back to global require function

If the require method on GeneratorContext fails to find the module to require relative to the Antora installation, it should fall back to using the global require function. This avoids complications when the Antora installation is split. What the GeneratorContext#require is then for is to resolve the module first inside the Antora installation (where the global require function may not be looking) before looking in the global path.

We can file a separate issue to enable strict mode to skip looking in the global scope.