source: Support complex pin, like key does.

source: Support complex pin, like key does.

This adds support for passing in a full pin declaration as the pin
parameter on a source declaration.

It keeps the old behaviour intact, you can still simply do `pin => '10'`
and it will pin on origin with a priority of 10.

Should that not be what you want you can now pass in a full pin
declaration instead. We make no assumptions here, whatever you pass in
will be passed through to pin as-is with the exception of the values for
`ensure` and `before` which are always overridden by us to ensure
everything keeps working as designed.