Fix a bug in the parser code that decides whether a given name should
    be considered exported or not. The function Lex::is_exported_name
    (which assumes that its input is a mangled name) was being called on
    non-mangled (raw utf-8) names in various places. For the bug in
    question this caused an imported package to be registered under the
    wrong name. To fix the issue, rename 'Lex::is_exported_name' to
    'Lex::is_exported_mangled_name', and add a new 'Lex::is_exported_name'
    that works on utf-8 strings.
    
    Fixes golang/go#27836.
    
    Reviewed-on: https://go-review.googlesource.com/137736

From-SVN: r264690