Cannot set runtimepath correctly

I am trying to install The Ultimate Vim Configuration on my Windows PC (from https://github.com/amix/vimrc ), and I can't get the runtimepath to work. I have a folder inside my home folder called "vim_runtime" and inside that folder are all the folders containing all the plugins and settings and such. The subfolders in the folder are: "autoload", "sources_forked", "sources_non_forked", "temp_dirs", and "vimrcs". My _vimrc file is as follows:

BEGIN DISPLAY _VIMRC:

cd $HOME
set runtimepath+=~/vim_runtime

source ~/vim_runtime/vimrcs/basic.vim
source ~/vim_runtime/vimrcs/filetypes.vim
source ~/vim_runtime/vimrcs/plugins_config.vim
source ~/vim_runtime/vimrcs/extended.vim

try
source ~/vim_runtime/my_configs.vim
catch
endtry

END DISPLAY _VIMRC

As I understand it, when you call "set runtimepath" on a folder all of its subfolders should be added to the runtimepath as well (correct me if I'm wrong on this). However, when I load gVim and view the runtimepath, only "~/vim_runtime" is getting added and none of the subfolders. However, the .vim files in the vimrcs/ folder are getting executed (sourced) correctly, because when I start gVim I get an error saying that it could not find the peaksea color scheme (line 37 of extended.vim). Which makes sense, because the folder that contains peaksea.vim (sources_forked/peaksea) is not in runtimepath. How do I fix this, other than manually adding every single subfolder and subfolder of the subfolder to runtimepath?

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