go.tools/importer: honor the client's TypeChecker.{Import,Error} values.
go.tools/importer: honor the client's TypeChecker.{Import,Error} values.

This requires us to make a copy of (not clobber) the supplied
config, and retain their Import hook separately so that it can
be wrapped by Importer.doImport.

Fixes bug 6562.

R=gri
CC=golang-dev
https://codereview.appspot.com/14523054

webfd/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,8limbs/gotools,8limbs/gotools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools