Swift: Support v3 file format, downgrade version mismatch to warning in the future

[jssblck]

Overview

Resolves ANE-1739:

• Downgrades version mismatch in the Package.resolved file to a warning, if parsing otherwise succeeds. If parsing fails because the actual shape of the data changes in a backwards-incompatible way, that will still be an error.
• Adds explicit support for v3, which is not different from v2 for our purposes.

Acceptance criteria

SwiftPM v3 files can be parsed.

Testing plan

I added an automated test for this and I am relying on that.

Risks

None

Metrics

None

References

Resolves: https://fossa.atlassian.net/browse/ANE-1739

Checklist

• I added tests for this PR's change (or explained in the PR description why tests don't make sense).
• If this PR introduced a user-visible change, I added documentation into docs/.
• If this PR added docs, I added links as appropriate to the user manual's ToC in docs/README.ms and gave consideration to how discoverable or not my documentation is.
• If this change is externally visible, I updated Changelog.md. If this PR did not mark a release, I added my changes into an # Unreleased section at the top.
• If I made changes to .fossa.yml or fossa-deps.{json.yml}, I updated docs/references/files/*.schema.json AND I have updated example files used by fossa init command. You may also need to update these if you have added/removed new dependency type (e.g. pip) or analysis target type (e.g. poetry).
• If I made changes to a subcommand's options, I updated docs/references/subcommands/<subcommand>.md.