Allow '.' for typename in annotation comment

[c9s]

Summary:

For types from other packages, we need to reference the typename with
package name. e.g., "ast.Function"

Related Issue: clipperhouse/gen#88

[c9s]

Use case code:

// +gen symtable:"SymTable[ast.Function]"
type FunctionSymTable map[string]*ast.Function

This fixes the lexer, but we need to let the go/ast works, I am currently tracing the code of type parsing

[c9s]

I will submit the external type support in another PR.

[c9s]

Some updates

• Unfortunately go/types.Checker didn't open much API that we can use. We need to pass go/types.Info to let Checker knows the external package.
• Also the check.rawExpr(&x, node, nil) method is not exported, that way even we copy the code, we still can't eval the external type.