Harcoded component of 'main' when reading DEB packages from pool

[ggprod]

This line: https://github.com/tarantool/mkrepo/blob/master/debrepo.py#L439

as well as this line: https://github.com/tarantool/mkrepo/blob/master/debrepo.py#L495

when processing index units mean that all index units would be associated with a 'main' component.

A better approach would be updating https://github.com/tarantool/mkrepo/blob/master/debrepo.py#L495 to instead extract the component from the path (like dist) and/or update https://github.com/tarantool/mkrepo/blob/master/debrepo.py#L439 to omit the /main suffix or make it a capture group for the component name