Remove required check of version field for CRAN and Swift

[calebbrown]

Currently the version for the Swift and CRAN ecosystems is validated as "required". If the version is missing for these ecosystems then validation fails.

According to https://github.com/package-url/purl-spec/blob/main/types/cran-definition.json and https://github.com/package-url/purl-spec/blob/main/types/swift-definition.json the "version" field is optional.

This change removes this check for these ecosystems.

[petergardfjall]

Thanks for the PR.

I'd like to incorporate both of these cases in #83, which adapts to code to pass the upstream (purl-spec) defined tests.

• The cran case is already handled.
• As for the swift case there are still upstream tests that require the presence of a version, so those need to be fixed first. I've submitted a PR for those: drop swift testcases that require version purl-spec#806