go/parser: resolve identifiers properly

Correctly distinguish between lhs and rhs identifiers
and resolve/declare them accordingly.

Collect field and method names in respective scopes
(will be available after some minor AST API changes).

Also collect imports since it's useful to have that
list directly w/o having to re-traverse the AST
(will also be available after some minor AST API changes).

No external API changes in this CL.

R=rsc, rog
CC=golang-dev
https://golang.org/cl/4271061

src/pkg/go/parser/interface.go

@@ -69,7 +69,7 @@ func ParseExpr(fset *token.FileSet, filename string, src interface{}) (ast.Expr,
 
 	var p parser
 	p.init(fset, filename, data, 0)
-	x := p.parseExpr()
+	x := p.parseRhs()
 	if p.tok == token.SEMICOLON {
 		p.next() // consume automatically inserted semicolon, if any
 	}