The parser populates all scopes for a given file (except
type-related scopes for structs, interfaces, and methods
of types) at parse time.

A new parser flag, DeclarationErrors, enables error messages
related to declaration errors (as far as it is possible to
provide them).

The resulting AST has all (non-field, non-method) identifiers
resolved that can be resolved w/o doing imports or parsing
missing package files.

The ast.File node contains the (partially complete)
package scope and a list of unresolved global identifiers.

All type-specific data structures have been removed from the AST.

The existing typechecker is functional but needs to be adjusted
(simplified) accordingly. Utility functions to resolve all
identifiers for a package (after handling imports and parsing
all package files) are  missing.

Unrelated changes:
- Rename typechecker/testdata files to that they are not considered
  by gofmt.
- Minor cleanups/simplifications.

Parses all .go files in src and misc without declaration errors.
Runs all tests. Changes do not affect gofmt output.

R=rsc
CC=golang-dev
https://golang.org/cl/4244049