tests/test_{convert,files}.py:
Ensure that instantiation of models using the `License` model are done
correctly.

repo_management/convert.py:
Change the docuemntation of `_desc_data_line_to_dicts()` to reflect,
that it may raise ValueError under certain conditions.
Change `_desc_data_to_model()` to try/except ValueError raised by
`_desc_data_line_to_dicts()` and raise a
`errors.REpomanagementValidationError` instead.

tests/test_convert.py:
Change tests expecting to raise `ValueError` to expect
`errors.RepoManagementValidationError` instead.

repo_management/convert.py:
Add the class `RepoDbFile` which is used to initialize a jinja
environment and allows to render the 'desc.j2' and 'files.j2' templates.
Add `_desc_data_line_to_dicts()` which allows for assigning the data
from a line in a 'desc' file to be assigned to a dict, that corresponds
to the line's designated data type.
Simplify `_desc_data_to_model()` by making use of
`_desc_data_line_to_dicts()`.
Change `_desc_data_to_model()` by guarding against pydantic
ValidationErrors and raising a RepoManagementValidationError instead.

tests/test_convert.py:
Add tests for `RepoDbFile`.

repo_management/convert.py:
Add private method `_transform_package_desc_to_output_package()` which
can be used to construct a `models.OutputPackage` from a
`models.PackageDesc` (and optionally a `models.PackageFiles`).
The method drops attributes from a dict representation of the
`models.PackageDesc` instance, that are not relevant for the
`models.OutputPackage`.

tests/test_convert.py:
Add parametrized test for `_transform_package_desc_to_output_package()`.

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()`.

tests/test_convert.py:
Add initial tests for the convert module by adding a parametrized test
for `_files_data_to_dict()`.