repo_management/defaults.py:
Add the IntEnmu `FieldType` which tracks the required type for an
attribute when converting from 'desc' or 'files' files to pydantic
models.
Extend the DESC_JSON and FILES_JSON globals by introducing a dict per
key, that describes the name and the required type, when converting to
pydantic models.

repo_management/convert.py:
Rename `_files_data_to_dict()` to `_read_files_data_to_model()` and
return a pydantic model (`models.Files`) instead of a custom dict.
Add `_desc_data_to_model()` which allows for conversion of a 'desc' data
file (represented as io.StringIO) to a pydantic model
(`models.PackageDesc`).

tests/test_convert.py:
Add another test for raising RuntimeError in
`_read_files_data_to_model()`.
Add a parametrized test for `_desc_data_to_model()`.