Review request for runtime support for exports

[Mandy Chung]

I have implemented the runtime support for "export" (see the grammar in [1] and [2])
that also replaces the "requires public" re-exports mechanism.

Jon, I modified a few javac files for the new exports() method added in
the ModuleInfo class.

Webrev at:
    http://cr.openjdk.java.net/~mchung/jigsaw/webrevs/exports-runtime

Changes include:
1. Context stores a map from a remote class to the supplying context.
2. Configuration stores the list of remote classes rather than remote packages.
     - This is to handle the case that a module A only exporting some public types
       but keep a few public types invisible and another module B re-exporting A
       using the wildcard.

module A {
      export a.A;  // a.B class is public but not exported
}

module B {
      requires A;
      export a.*;  // what classes should module B re-export?
}

What types should B re-export?

I think module B should only propagate the visible exported types from A
that's what I currently implement.

3. Linker determines if a local class is exported by checking it against the
     ModuleExport attribute of its module info.  To determine if a remote class
     is re-exported from a supplying context, it will check against the exports
    defined in all module infos in that context.

4. PathLinker has to do some analysis to find all remote contexts supplying
     public types directly and indirectly.  The way I implemented is to compare
     the exports from a context and a given supplier (i.e. the required module's
     context); it re-exports the types from the supplier when there is one matching
     export.

During testing, I found that the resolver detects the split-package issue
only if more than one module contains a visible type in the same package.
In other word, if module A exports a public type a.A and module B has
a public type a.B but not exported, when a module Foo requires both
A and B, what is the expected behavior for the resolver?   How should
"local" and "permits" be used in this case?

I added a test case (split-package) in _Configurator.java test for what
described above.  The resolution succeeded in the current implementation.

Thanks
Mandy

[1] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2011-March/001201.html
[2] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2011-April/001225.html