go-tools patch 84 from Alan Donovan
go.tools/ssa: use go/types.LookupFieldOrMethod, and simplify.

Added tests.
Merge revision 85 from Alan Donovan
go.tools/ssa: some renamings.

- Prog.Files -> Fset
- Prog.Packages -> PackagesByPath
- Prog.Builtins -> builtins
- Package.Types -> Object