Schema v1.4, include find directive.

For #376
ModuleInstallDescriptors now have a `find` type.

- `ModuleInstallDescriptor.DefaultInstallStanza` replaces the old one
  in ModuleInstaller.
- `ModuleInstallDescriptor` serialisation options are much more
  explicit.
- `ConvertFindToFile` allows generation of a `file` stanza from a `find`
  stanza and zipfile. (This is our old DefaultInstallStanza repurposed.)
- Tests updated.
- For #376