Simplify per @tarjoilija's feedback

Use source in `${ZGEN_INIT}` instead of -zgen-source. Fixes tarjoilija#7, with the
welcome side effect of being 11% faster.

zgen.zsh

@@ -87,7 +87,7 @@ zgen-save() {
  echo "# This file will be overwritten the next time you run zgen save" >> "${ZGEN_INIT}"
  echo "#" >> "${ZGEN_INIT}"
  for file in "${ZGEN_LOADED[@]}"; do
- echo "-zgen-source \"$file\"" >> "${ZGEN_INIT}"
+ echo "source \"$file\"" >> "${ZGEN_INIT}"
  done
 
  # Set up fpath
@@ -96,14 +96,6 @@ zgen-save() {
  echo "# Add our plugins and completions to fpath">> "${ZGEN_INIT}"
  echo "#" >> "${ZGEN_INIT}"
  echo "fpath=(\$fpath $ZGEN_COMPLETIONS )" >> "${ZGEN_INIT}"
-
- # Reset
- echo >> "${ZGEN_INIT}"
- echo "#" >> "${ZGEN_INIT}"
- echo "# Clear ZGEN_LOADED and ZGEN_COMPLETIONS per #7">> "${ZGEN_INIT}"
- echo "#" >> "${ZGEN_INIT}"
- echo "ZGEN_LOADED=()" >> "${ZGEN_INIT}"
- echo "ZGEN_COMPLETIONS=()" >> "${ZGEN_INIT}"
 }
 
 zgen-completions() {