python: Optimize completion for loading lazily on demand

Move `pip` to separate completion definition delegating to pip to do
all the completion work.

Since the completion is loaded lazily on-demand, it avoids the
performance overhead during Zsh initialization.

Implementation note:
The helper function `_pip_completion` implementation is based on the
official pip completion function (which can be generated with
`pip completion --zsh`) adhering to the newer compsys style.

See: 'man zshcompsys' for more details.

modules/python/functions/_pip

@@ -0,0 +1,19 @@
+#compdef -P pip[0-9.]#
+#autoload
+
+#
+# Pip completion, delegating to pip to do all the completion work.
+#
+# Authors:
+#   Indrajit Raychaudhuri <irc@indrajit.com>
+#
+
+if (( $+commands[$words[1]] )); then
+
+  function _pip_completion {
+    compadd -- $( COMP_WORDS="$words[*]" COMP_CWORD=$(( CURRENT - 1 )) \
+                  PIP_AUTO_COMPLETE=1 $words[1] 2>/dev/null )
+  }
+  _pip_completion "$@"
+
+fi