Support property ${baseurl} to get RFC-compliant URL of project base directory

If people currently need a URL to their project base directory or any file within, they need to write file://$\{basedir/}}. The problem about this approach is that it doesn't deliver a RFC-compliant URL since characters are not properly percent-encoded. Also, the exact number of slashes between file: and {{$

}} depends on the OS (e.g. file:///C:/user/ and file:///home/user, Unix paths have a leading slash by themselves, Windows not). This makes it currently impossible to configure plugins that expect a URL as input and do strict URL parsing.

For this reason, I suggest to support an additional property {{$

}} with the value

  baseurl = new File( basedir ).toURI().toString()

for POM interpolation.

Some day, when Maven/Wagon itself handle percent-encoded file: URLs correctly (WAGON-111), this property could also be used to define local deployment repos like we commonly use for testing.