Handle multiple SRPM NVRs with the same `name` in Lightblue class.
Before this commit, when `srpm_nvrs` contained multiple NVRs with
the same name, the `filter_out_images_with_lower_srpm_nvr` method
wrongly picked up only single NVR. This was caused by a fact that
we use `dict` in `{srpm_name: srpm_nvr}` to store this data.
In this commit, this has been changed and we now use `dict` in the
`{srpm_name: [srpm_nvr1, srpm_nvr2, ...]` format. This allows
handling this case and fixes the `filter_out_images_with_lower_srpm_nvr`
method.