Simplify conversion from package-name/ID to port-name/ID.

* adjust getPortIdForPackageId() and getPortNameForPackageName()
  to check if the given parameter is already valid, simplifying
  the loop, too
* drop explicit checks before invocation of getPortIdForPackageId()
Automatic whitespace cleanup - no functional change intended.
Cleanup: drop now superfluous method call.
Fix provides of source packages.

* Use port name, not name of last added package when creating the 
  source package.
* Let the source package provide its own name (e.g. 'libpcre_source')
  instead of repeating what the base package provides.