Extend path substitution to access OSGI bundles.

[mypsycho]

As Eclipse platform is also a repository, pydev projects could access contained bundles and use it to share scripts with the proper syntax.

In previous implementation, variables using an expression with ':' are not supported. Substitution can use a well-known prefix 'bundle:' to fetch path of OSGI bundle. Eclipse users would be able to use P2 repositories to easily share scripts.