repo_management/files.py:
Add the private method `_extract_db_member_package_name()` which can be
used to extract the package name from the name of a member of repository
database.
Rename `_read_db_file_member()` to `_db_file_member_as_model()` and
change the method to yield a `models.RepoDbMemberData` instead of a
`tarfile.TarInfo` to provide actual data (and required context) to
callers of the method.

tests/test_file.py:
Add parametrized test for `_extract_db_member_package_name()`.
Change test for `_db_file_member_as_model()` to assert the correct
return type.

repo_management/files.py:
Add iterator method `_read_db_file_member()` to yield the members of a
provided tarfile.TarFile.
The members are filtered by name using a regular expression, which can
be overridden.

tests/test_file.py:
Change `create_db_file()` to also create 'desc' and 'files' files below
the created temporary directories.
Assert, that all tested methods return a value.
Add test for `_read_db_file_member()`.

repo_management/files.py:
Add `_read_db_file()` to open a db file and return it as
tarfile.Tarfile. The method assumes gzip-compressed databases by
default, but can be used with other compression algorithms.

tests/test_file.py:
Add fixtures and tests for `_read_db_file()`.