Support MacPorts as external provider. (#1404)

* src/alire/alire-origins-deployers-system-macports.adb: new.
  * src/alire/alire-origins-deployers-system-macports.ads: new.
  * src/alire/alire-origins-deployers-system.adb
    (context): add Alire.Origins.Deployers.System.Macports.
    (Platform_Deployer): Platforms.Macports => System.Macports.Deployer.
  * src/alire/alire-platforms.ads
    (Distributions): add Macports.
    (Package_Managers): likewise.
    (Distro_Manager): Macports => Macports.
  * src/alire/alire-utils-tools.adb (System_Package_For_Tool): add Macports,
      same as the other distributions.
  * src/alire/os_macos/alire-platforms-current__macos.adb:
    (context): add GNAT.OS_Lib.
    (Port_Access): new, access to Macports' 'port' executable.
    (Macports_Present): new, true if 'port' was found.
    (Detected_Distribution): first, check for Homebrew_Present, then, for
      Macports_Present. Return the corresponding Distribution if found.