Reorganize file structure for common components

[taesungh]

Noticed in #498 we have a apps/site/src/components directory which is mostly unused because currently components are stored in apps/site/src/lib/components instead. Conceptually, I see no reason to nest the components one level deeper, and it would be simpler to store them in the higher-up location. It is also mildly annoying to have this detachment because in the VS Code Source Control view, the changed files are listed alphabetically by path, and any modifications to the form components would be listed further away from changes to form pages in src/app when there are also changes to files in src/lib/admin.

This will require updating all of the corresponding imports which should make each of them slightly shorter as well. This does not have to happen all it once, and it would help to have a gradual migration in multiple phases. This would also be a good opportunity to further shorten the imports by adding index files with reexports, e.g.

import Button from "@/components/Button";

or

import { Button } from "@/components";

instead of

import Button from "@/lib/components/Button/Button";