Switch to pure-ruby which (actually windows-like where)

[yaauie]

This is a pre-requisite for #2. I may want to also be able to supply a PATH-like string manually instead of relying on ENV['PATH']

    # Analog of Windows `where` command, or a `which` that finds *all*
    # matching executables on the supplied path.
    # @param cmd [String] - the command to find
    # @return [Enumerable<String>] - the executables found, lazily.
    def find_executables(cmd)
      return enum_for(:find_executables, cmd) unless block_given?

      exts = ENV['PATHEXT'] ? ENV['PATHEXT'].split(';') : ['']
      ENV['PATH'].split(File::PATH_SEPARATOR).each do |path|
        exts.each do |ext|
          exe = File.join(path, "#{cmd}#{ext}")
          yield exe if File.executable? exe
        end
      end
    end