Fix Poetry lock optional dependency parsing

[Mars-60]

Fixes #5294

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)

Description

PoetryLockHandler incorrectly reported resolved Poetry lockfile packages as optional even when the package had optional = false in poetry.lock.

The fix maps Poetry's optional field to ScanCode's is_optional field:

is_optional = package.get("optional", False)