Helper for 'local' libs versions

This is an object which provide the functionality to create a
local::lib 'helper' script in either the currently loaded local::lib
environment or in a target directory of choice. i
By default the script is called localenv and can be used to invoke
a command under the local::lib which it was built against.
For example, assume you build a local::lib like so:

  cpanm --local-lib ~/mylib App::local::lib::helper