requires/types/snap: add 'version' requirement type

[mustafakemalgilor]

some of the scenarios may need "version" conditions instead of revisions. this patch does the following:

• adds a new "version" criterion to snap requirement
• both "revision" and "version" now uses DPKGVersion helper to perform the check
• "revision" now has an explicit name instead of being inferred from "min"/"max" being present
• simplified the package_info_matches() function
• updated existing scenarios to make them comply with the new syntax
• added unit tests for the new snap "version" criterion
• updated requirement_types.rst to reflect the changes to the docs
• added op name validation to DPKGVersion.is_version_within_ranges()