refactored TPM plugins discovery, resolves #671

lift the limitation that allows plugins to only be declared from the main
`.local` customization file

from now on, spawn an ephemeral tmux server/client pair to discover plugins with
a mix of source-file -nvq and a recursive shell function

the upside of the new approach is that it respects tmux conditionals and we only
consider set -g @plugin that are really evaluated by tmux, e.g. within %if,
%elif, %else blocks

---
currently, source-file -v doesn't apply the -v flag to subsequent source-file
commands being evaluated, so we use -n and recurse ourselves

trying to (ab)use the after-set-option hook to accumulate all the evaluations of
set -g @plugin into a variable was promising, but is not immune to configuration
files that use set-(hook|option) -gu after-set-option to clear the hook, which
makes plugin discovery fail partially or even entirely

.tmux.conf

@@ -164,6 +164,8 @@ run 'cut -c3- "$TMUX_CONF" | sh -s _apply_configuration'
 # # exit the script if any statement returns a non-true return value
 # set -e
 #
+# unset SHELL
+#
 # unset GREP_OPTIONS
 # export LC_NUMERIC=C
 # # shellcheck disable=SC3041
@@ -603,7 +605,7 @@ run 'cut -c3- "$TMUX_CONF" | sh -s _apply_configuration'
 #     battery_status="$battery_status_charging"
 #   fi
 #
-#   tmux set -g '@battery_status' "$battery_status"
+#   tmux set -g '@battery_status' "$battery_status" >/dev/null 2>/dev/null
 # }
 #
 # _pane_info() {
@@ -916,7 +918,7 @@ run 'cut -c3- "$TMUX_CONF" | sh -s _apply_configuration'
 #   if _is_true "$tmux_conf_preserve_stock_bindings"; then
 #     probe_socket="$(dirname "$TMUX_SOCKET")/tmux-stock-bindings-$$"
 #     TMUX_SOCKET="$probe_socket" tmux -f /dev/null list-keys >> "$cfg"
-#     rm -f "%probe_socket"
+#     rm -f "$probe_socket"
 #   fi
 #
 #   # tmux 3.0 doesn't include 02254d1e5c881be95fd2fc37b4c4209640b6b266 and the
@@ -1619,8 +1621,48 @@ run 'cut -c3- "$TMUX_CONF" | sh -s _apply_configuration'
 #   fi
 #   mkdir -p "$TMUX_PLUGIN_MANAGER_PATH"
 #
+#   __discover_plugins() (
+#     probe_socket="$(dirname "$TMUX_SOCKET")/tmux-discover-plugins-$$"
+#     TMUX_SOCKET="$probe_socket" tmux -f /dev/null start-server \; set-option exit-empty off
+#     ___discover_plugins() {
+#       depth=$((${depth:-0} + 1))
+#       IFS=${_IFS:-$IFS}
+#       [ "$depth" -le 100 ] || return
+#       for current_file in "$@"; do
+#         current_file="$(cd "${current_file%/*}" 2>/dev/null && pwd)/${current_file##*/}" || continue
+#         while IFS= read -r line; do
+#           if plugin=$(printf '%s\n' "$line" | perl -s -n -E 'print if s/^set-option\s+-g\s+\@plugin\s+//g or die' 2>/dev/null); then
+#             discovered_plugins="${discovered_plugins}${discovered_plugins:+ }${plugin}"
+#           elif next_files=$(printf '%s\n' "$line" | perl -s -n -E 's/(?<!\@)current_file/\@current_file/g ; print if s/^source(?:-file)?(?:\s+-[qF]+)*\s*(.+?)$/\1/g or die' 2>/dev/null); then
+#             next_files=$(TMUX_SOCKET="$probe_socket" tmux -f /dev/null \
+#               set -g @current_file "$current_file" \; \
+#               display -pF "$next_files")
+#
+#             _IFS="$IFS"
+#             IFS=$(printf '\n\nx')
+#             IFS=${IFS%?}
+#             # we don't want quoting here as we want wildcard expansion
+#             # shellcheck disable=SC2046
+#             ___discover_plugins $(printf '%s\n' "$next_files" | xargs printf '%s\n\n')
+#           fi
+#         done << EOF
+# $(TMUX_SOCKET="$probe_socket" tmux -f /dev/null source -nvq "$current_file" | perl -s -n -E 'print if s/^$current_file:\d+:\s*(set-option\s+-g\s+\@plugin\s+|source-file)/\1/g' -- -current_file="$current_file")
+# EOF
+#       done
+#     }
+#     ___discover_plugins "$@"
+#     TMUX_SOCKET="$probe_socket" tmux -f /dev/null kill-server
+#     rm -rf "$probe_socket"
+#     printf '%s\n' "$discovered_plugins"
+#   )
+#
 #   tpm_plugins=$(tmux show -gvq '@tpm_plugins' 2>/dev/null)
-#   if [ -z "$(tmux show -gv '@plugin' 2>/dev/null)" ] && [ -z "$tpm_plugins" ]; then
+#   tpm_plugins=$(cat << EOF | tr ' ' '\n' | awk '/^\s*$/ {next;}; !seen[$0]++ { gsub(/^[ \t]+/,"",$0); gsub(/[ \t]+$/,"",$0); print $0 }'
+#     $tpm_plugins
+#     $(__discover_plugins "$TMUX_CONF_LOCAL")
+# EOF
+#   )
+#   if [ -z "$tpm_plugins" ]; then
 #     if _is_true "$tmux_conf_uninstall_plugins_on_reload" && [ -d "$TMUX_PLUGIN_MANAGER_PATH/tpm" ]; then
 #       tmux display 'Uninstalling tpm and plugins...'
 #       tmux set-environment -gu TMUX_PLUGIN_MANAGER_PATH
@@ -1629,11 +1671,6 @@ run 'cut -c3- "$TMUX_CONF" | sh -s _apply_configuration'
 #     fi
 #   else
 #     if [ "$(command tmux display -p '#{pid} #{version} #{socket_path}')" = "$("$TMUX_PROGRAM" display -p '#{pid} #{version} #{socket_path}')" ]; then
-#       tpm_plugins=$(cat << EOF | tr ' ' '\n' | awk '/^\s*$/ {next;}; !seen[$0]++ { gsub(/^[ \t]+/,"",$0); gsub(/[ \t]+$/,"",$0); print $0 }'
-#         $tpm_plugins
-#         $(awk '/^[ \t]*set(-option)?.*[ \t]@plugin[ \t]/ { gsub(/'\''/, ""); gsub(/'\"'/, ""); print $NF }' "$TMUX_CONF_LOCAL" 2>/dev/null)
-# EOF
-#       )
 #       tmux set-environment -g TMUX_PLUGIN_MANAGER_PATH "$TMUX_PLUGIN_MANAGER_PATH"
 #       tmux set -g '@tpm_plugins' "$tpm_plugins"
 #