- created new module for API model types `eark_validator.model` that contains types for:
  - `Level` (requirements level);
  - `PackageDetails`;
  - `Severity` (of test result);
  - `StructResults` (for structural validation results);
  - `StructStatus` (for structural validation status);
  - `TestResult` (for validation test results); and
  - `ValidationReport` (for final validation report);
- refactored structural validation to use new types;
- small fix to schematron test for `CSIP11` in `mets_metsHdr_rules.xml`;
- new `eark_validator.packages` module for package validation;
- introduced a `PackageHandler` type to start abstraction of package parsing (replaces `STRUCT.ArchivePackageHandler`);
- introduced a `ValidationReport` type to handle final aggregation of validation results;
- added missing `__init__.py` files with appropriate commenting;
- removed some defunct types;
- removed unused imports;
- introduced light use of pydantic, better use to come; and
- unit test improvements and fixes.

FIX: Pylint errors.

CI: Python build matrix, 3.9 to 3.12