Ask pyenv whether pip exists if installed

Pyenv will install shims for commands that exist in any interpreter,
even if it is not the current one. This means that a command may
technically exist, but when executed will tell the user to try a
different interpreter. The original check for pip in the Python module
can fail for this reason, in particular on Ubuntu 20.04.

This change checks with pyenv whether pip really exists in the current
interpreter to work around this problem.

Fix bug in pip command detection

modules/python/init.zsh

@@ -147,27 +147,35 @@ if (( $+VIRTUALENVWRAPPER_VIRTUALENV || $+commands[virtualenv] )) && \
 fi
 
 # Load PIP completion.
-if (( $#commands[(i)pip(|[23])] )); then
-  cache_file="${XDG_CACHE_HOME:-$HOME/.cache}/prezto/pip-cache.zsh"
-
-  # Detect and use one available from among 'pip', 'pip2', 'pip3' variants
+# Detect and use one available from among 'pip', 'pip2', 'pip3' variants
+if [[ -n "$PYENV_ROOT" ]]; then
+  for pip in pip{,2,3}; do
+    pip_command="$(pyenv which "$pip" 2>/dev/null)"
+    [[ -n "$pip_command" ]] && break
+  done
+  unset pip
+else
   pip_command="$commands[(i)pip(|[23])]"
+fi
+if [[ -n "$pip_command" ]]; then
+  cache_file="${XDG_CACHE_HOME:-$HOME/.cache}/prezto/pip-cache.zsh"
 
   if [[ "$pip_command" -nt "$cache_file" \
         || "${ZDOTDIR:-$HOME}/.zpreztorc" -nt "$cache_file" \
         || ! -s "$cache_file" ]]; then
     mkdir -p "$cache_file:h"
     # pip is slow; cache its output. And also support 'pip2', 'pip3' variants
-    $pip_command completion --zsh \
+    "$pip_command" completion --zsh \
       | sed -e "s/\(compctl -K [-_[:alnum:]]* pip\).*/\1{,2,3}{,.{0..9}}/" \
       >! "$cache_file" \
       2> /dev/null
   fi
 
   source "$cache_file"
 
-  unset cache_file pip_command
+  unset cache_file
 fi
+unset pip_command
 
 # Load conda into the shell session, if requested
 zstyle -T ':prezto:module:python' conda-init