feat: convert sourceDirectory to a template eligible config

[setchy]

Changes

Convert sourceDirectory to be a template eligible field.

Demonstration repo: https://github.com/setchy/renovate-atlaskit-changelog-two

Context

Help with monorepo package updates where there is a source folder pattern based upon the package name.

For example using https://bitbucket.org/atlassian/atlassian-frontend-mirror and their design-system packages

{
  "extends": ["packages:atlaskit"],
  "sourceUrl": "https://bitbucket.org/atlassian/atlassian-frontend-mirror",
  "sourceDirectory": "design-system/{{ lookup (split packageName '/') 1 }}"
}

As opposed to setting source directory for each of the ~50+ packages, ie 😅 https://github.com/setchy/renovate-atlaskit-changelog/blob/main/renovate.json

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 39.31.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 39.31.0

Your semantic-release bot 📦🚀