RFR 8064924: Update java.net.URL to work with modules

[Chris Hegarty]

This is phase 1, of getting java.net.URL work with modules. 

Being able to effectively specify URL protocol handler factories as fully qualified class names, through the 'java.protocol.handler.pkgs' system property is problematic. It requires the protocol handler factory implementation class to be public and accessible, as the current implementation tries to instantiate it through core reflection. Since the protocol handler factory must be an implementation of a URLStreamHandlerFactory, it lends itself to being retrofitted with a ServiceLoader lookup. Note, the 'java.protocol.handler.pkgs' system property mechanism predates ServiceLoader. URL protocol handlers would most likely have used service loader if it were available at the time. 

Some URL protocol handlers are fundamental to the platform itself, like 'file' and 'jar', it is not appropriate to attempt a service loader lookup for these, as they may lead to recursive initialization issues. However, Java Plugin, Webstart, and possibly other containers, do override the 'jar' handler. A new API will be provided for this purpose. Providing an API solution should not interfere with system initialization as it will only be called after the system comes up and user code is executing. 

The 'file' protocol handler factory will no longer be overridable, and the system property will no longer be consulted.

Webrev and spec diffs:
  http://cr.openjdk.java.net/~chegar/8064924/00/

I want to agree the approach and spec first, so that the spec changes can be submitted for approval. A comprehensive test will be added before the code changes are finalised.

-Chris.