Flattened schema file names are too long

Apparently there are use cases where the flattened schema files need to be written to optical media but cannot have rock ridge extensions enabled, so filenames are limited to 64 characters and are truncated if longer. This obviously poses an issue with the flattened schemas and the naming convention of converting

```
/com/acme/xsd/schema.dfdl.xsd
```
to
```
com__acme__xsd__schema.dfdl.xsd
```

While preserving the directory layout is nice, we probably need to switch to using a hash for that portion of the filename or some other way of shortening the filenames.

Start by tracing how flattened schema paths are converted into output filenames. Check the current directory-to-name convention against the 64-character optical-media limit and determine how shortened names remain usable and distinguishable. Done means flattened schema filenames no longer exceed the limit without creating collisions.

Written by the indexing model from the issue text.