BF: freeze_versions - do not hardcode .sing extension, make overall more flexible, robust and test

[yarikoptic]

see added tests for use cases -- can specify incomplete version or an entire filename with extension now.

ref: https://github.com/ReproNim/containers/issues/64#issuecomment-1498510384

@stebo85 - if you have a pick/provide feedback - would be appreciated.