Support for file urls

[ghenzler]

File urls are generally supported for maven repositories: https://maven.apache.org/guides/introduction/introduction-to-repositories.html#internal-repositories

Using file urls with artifacts provided in git repo itself can be useful, especially if it is only about parent poms. The big advantage is that

• No remote repository on the internet has to be maintained
• It can also be used for development team members that don't have access to such a repository

For the case the directory .mvn/repository exists in project structure (similar path as ~/.m2/repository), it should automatically be registered as repository. For other location a URL like file://${maven.multiModuleProjectDirectory}/artifacts/vendor1 or file://${maven.multiModuleProjectDirectory}/artifacts/vendor2 shall be supported (automatically replacing ${maven.multiModuleProjectDirectory} with the reactor root.