Added default_os field to PackageManagers enum elements.

This is useful to determine which is the default set of scripts that
should be used by the installer depending on the detected installed
package manager.

platypush/install/scripts/arch/PKGCMD

@@ -1 +1 @@
-pacman -S --noconfirm
+pacman -S --noconfirm --needed

platypush/utils/manifest.py

@@ -55,6 +55,11 @@ class PackageManager:
 
     executable: str
     """ The executable name. """
+    default_os: str
+    """
+    The default distro whose configuration we should use if this package
+    manager is detected.
+    """
     install: Iterable[str] = field(default_factory=tuple)
     """ The install command, as a sequence of strings. """
     uninstall: Iterable[str] = field(default_factory=tuple)
@@ -70,18 +75,21 @@ class PackageManagers(Enum):
         executable='apk',
         install=('apk', 'add', '--update', '--no-interactive', '--no-cache'),
         uninstall=('apk', 'del', '--no-interactive'),
+        default_os='alpine',
     )
 
     APT = PackageManager(
         executable='apt',
         install=('apt', 'install', '-y'),
         uninstall=('apt', 'remove', '-y'),
+        default_os='debian',
     )
 
     PACMAN = PackageManager(
         executable='pacman',
-        install=('pacman', '-S', '--noconfirm'),
+        install=('pacman', '-S', '--noconfirm', '--needed'),
         uninstall=('pacman', '-R', '--noconfirm'),
+        default_os='arch',
     )
 
     @classmethod