match unused imports where path and package name differ

When the import path and package don't match, Go emits an error a la

  imported and not used: "sandbox/foo_bar" as bar
fix typo
handle empty "import ()" in go-goto-imports
send the correct content-length to the playground

The url package uses (length) to determine the content length. For
multibyte sequences, that returns an incorrect result unless we
explicitly hand individual bytes to it.