repod/files/__init__.py:
Do not import top-level from repod.repo.management and repod.convert to
circumvent circular import.

repod/files/package.py:
Change `PackageV1` to also derive from the common models `CSize`,
`FileName`, `Md5Sum`, `PgpSig` and `Sha256Sum`, as these fields are
required in the context of the management repository.
Change `Package.from_file()` to accept a signature file as second
argument and to also populate the `csize`, `filename`, `md5sum`,
`pgpsig` and `sha256sum` fields.

repod/repo/management/outputpackage.py:
Add `OUTPUT_PACKAGE_VERSIONS` to track required and optional fields for
the derivates of OutputPackage.
Add the `OutputPackage.from_package()` classmethod to create a derivate
of OutputPackage from a `Package`.
Add the `OutputPackageBase.from_package()` classmethod to create a
derivate of OutputPackage from a list of `Package` instances.

tests/conftest.py:
Add session scoped `BuildinfoV1`, `BuildInfoV2`, `PkgInfoV1`,
`PkgInfoV2` and `PackageV1` fixtures.
Change `default_package_file` fixture to also return a dummy signature
file alongside the package file.

tests/files/test_common.py:
Adapt tests to changed `default_package_file` fixture.

tests/files/test_package.py:
Adapt tests to changes in `PackageV1` and `Package.from_file()`
behavior.

tests/repo/management/test_outputpackage.py:
Add tests for `OutputPackage.from_package()` and
`OutputPackageBase.from_package()`.