Revert "remove unnecessary autoloads"

Without these autoloads, invoking `magit-find-file-completing-read`
errors out with:

```
Symbol's function definition is void: magit-get-top-dir
```

This reverts commit b8390c7e5da39dea7fb9d59089a1cdd610e7e659.
Version 2.0.0

Significant refactor, including:

- removal of `magit-find-file-skip-images` - it's been replaced
  with a Customize option called `magit-find-file-ignore-extensions`
  which takes a list
- now includes `dash` (replaces use of `cl-lib`)
- remove argument to internal function `magit-find-file-files`
- inclusion in `magit-extensions` package.el group
- documentation and autoload fixes