Fix many subtle issues regarding on-demand loading etc.

- On-demand loading
    - Fix loading of unwanted files (e.g. colors/*.vim, syntax/*.vim, etc.)
- Filetyp-based on-demand loading
    - Load `after/ftdetect` as well
    - Make sure indent files are loaded by invoking
      `doautocmd filetypeindent FileType`
- Ensure plugin loaded when it was added after Vim started
- Do not reload $MYVIMRC after installtion/update
    - Instead simply call plug#end()

test/run

@@ -14,6 +14,23 @@ if [ ! -d fzf-staged ]; then
   git clone https://github.com/junegunn/fzf.git fzf-staged
 fi
 
+make_dirs() {
+  mkdir -p "$1"
+  cd "$1"
+  mkdir -p autoload colors ftdetect ftplugin indent plugin syntax
+  for d in *; do
+    cat > $d/xxx.vim << EOF
+    " echom expand('<sfile>')
+    let g:xxx = get(g:, 'xxx', [])
+    call add(g:xxx, '${1:4}/$d')
+EOF
+  done
+  cd - > /dev/null
+}
+
+make_dirs xxx/
+make_dirs xxx/after
+
 cat > /tmp/mini-vimrc << VIMRC
 set rtp+=vader.vim
 source $PLUG_SRC