go/types: Moving from *ast.Objects to types.Objects (step 1).

The existing type checker was relying on augmenting ast.Object
fields (empty interfaces) for its purposes. While this worked
for some time now, it has become increasingly brittle. Also,
the need for package information for Fields and Methods would
have required a new field in each ast.Object. Rather than making
them bigger and the code even more subtle, in this CL we are moving
away from ast.Objects.

The types packge now defines its own objects for different
language entities (Const, Var, TypeName, Func), and they
implement the types.Object interface. Imported packages
create a Package object which holds the exported entities
in a types.Scope of types.Objects.

For type-checking, the current package is still using ast.Objects
to make this transition manageable. In a next step, the type-
checker will also use types.Objects instead, which opens the door
door to resolving ASTs entirely by the type checker. As a result,
the AST and type checker become less entangled, and ASTs can be
manipulated "by hand" or programmatically w/o having to worry
about scope and object invariants that are very hard to maintain.

(As a consequence, a future parser can do less work, and a
future AST will not need to define objects and scopes anymore.
Also, object resolution which is now split across the parser,
the ast, (ast.NewPackage), and even the type checker (for composite
literal keys) can be done in a single place which will be simpler
and more efficient.)

Change details:
- Check now takes a []*ast.File instead of a map[string]*ast.File.
It's easier to handle (I deleted code at all use sites) and does
not suffer from undefined order (which is a pain for testing).
- ast.Object.Data is now a *types.Package rather then an *ast.Scope
if the object is a package (obj.Kind == ast.Pkg). Eventually this
will go away altogether.
- Instead of an ast.Importer, Check now uses a types.Importer
(which returns a *types.Package).
- types.NamedType has two object fields (Obj Object and obj *ast.Object);
eventually there will be only Obj. The *ast.Object is needed during
this transition since a NamedType may refer to either an imported
(using types.Object) or locally defined (using *ast.Object) type.
- ast.NewPackage is not used anymore - there's a local copy for
package-level resolution of imports.
- struct fields now take the package origin into account.
- The GcImporter is now returning a *types.Package. It cannot be
used with ast.NewPackage anymore. If that functionality is still
used, a copy of the old GcImporter should be made locally (note
that GcImporter was part of exp/types and it's API was not frozen).
- dot-imports are not handled for the time being (this will come back).

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7058060