Use XDG-compatible plugin path when available

If the `tmux.conf` file is found at a XDG_CONFIG_HOME, use this
directory to store the plugins instead of `$HONE/.tmux/`.

This is only effective if the user has not overridden the value of
`TMUX_PLUGIN_MANAGER_PATH` in their configuration file.

tpm

@@ -21,8 +21,17 @@ tpm_path_set() {
 	tmux show-environment -g "$DEFAULT_TPM_ENV_VAR_NAME" >/dev/null 2>&1
 }
 
+# Check if configuration file exists at an XDG-compatible location, if so use
+# that directory for TMUX_PLUGIN_MANAGER_PATH. Otherwise use $DEFAULT_TPM_PATH.
 set_default_tpm_path() {
-	tmux set-environment -g "$DEFAULT_TPM_ENV_VAR_NAME" "$DEFAULT_TPM_PATH"
+	local xdg_tmux_path="${XDG_CONFIG_HOME:-$HOME/.config}/tmux"
+	local tpm_path="$DEFAULT_TPM_PATH"
+
+	if [ -f "$xdg_tmux_path/tmux.conf" ]; then
+		tpm_path="$xdg_tmux_path/plugins/"
+	fi
+
+	tmux set-environment -g "$DEFAULT_TPM_ENV_VAR_NAME" "$tpm_path"
 }
 
 # Ensures TMUX_PLUGIN_MANAGER_PATH global env variable is set.