Move garden to its own directory and allow loading flowers.

This creates a kivy/garden directory and allows one to install flowers there directly using garden install --kivy flower. The corresponding PR in garden is https://github.com/kivy-garden/garden/pull/5.

I really dislike the hackishness of installing and redirecting to .kivy/garden so this allows you to install directly to kivy/garden and hopfully for e.g. pyinstaller to discover these mods naturally. It doesn't change any old functionality, just adds a new option.