Query with file system modules

[benweaver]

There appears to be no way to query MarkLogic specifying the file system as modules database, and if a ML app-server is configured with "(file system)" as the modules database, there is no way to query it with a user that doesn't have the xdmp-eval-modules-change permission.

Both would be solved by, e.g. interpreting "modules_database: 0" (or some other defined string) as a request to use the file system (i.e. create the option without calling through the xdmp:database function).

It might also be possible to remove the default "Modules", so that specifying "modules_database: null" will ensure the request uses whatever modules database is defined on the app-server. This doesn't allow users to choose the file system, but does allow all users to defer responsibility for choosing a modules db, which would be enough in some cases (and, as it happens, for me).

Happy to raise a PR, just wanted to get your opinion, in case if there are other concerns I'm not aware of.