Support partial base classes

[latonz]

Mapperly supports base classes with mapping methods which have a body.
E.g.

class DateMapper
{
    protected DateOnly DateTimeToDateOnly(DateTime dt) => DateOnly.FromDateTime(dt);
}

[Mapper]
partial class DtoMapper : DateMapper
{
    public partial CarDto ToDto(Car car);
}

In this sample, when the DtoMapper needs to map a DateTime to a DateOnly Mapperly will use the DateMapper.DateTimeToDateOnly method.

However if the DateMapper is itself a Mapper with partial methods, these won't be re-used. This should be improved.
E.g.

[Mapper]
partial class BaseMapper
{
    protected partial BaseDto ToBaseDto(BaseObject source);
}

[Mapper]
partial class DtoMapper : BaseMapper
{
    public partial CarDto ToDto(Car car);
}

If the DtoMapper needs to map a property of source type BaseObject to target type BaseDto it should re-use the already built mapping BaseMapper.ToBaseDto, but this currently does not work.