🐛 Move facebook-loader.js into its component directory

[rsimha]

This PR moves facebook-loader.js into its component directory to fix these (currently silenced) type checking errors, which stem from the fact that extension code is expected to live in a directory of the format amp-foo/0.x/. See #23084 (comment) for more details.

Type checking failed:
extensions/amp-facebook-comments/0.1/amp-facebook-comments.js:17: ERROR - [JSC_JS_MODULE_LOAD_WARNING] Failed to load module "../../amp-facebook/facebook-loader"
import {createLoaderLogo} from '../../amp-facebook/facebook-loader';
^

extensions/amp-facebook-page/0.1/amp-facebook-page.js:17: ERROR - [JSC_JS_MODULE_LOAD_WARNING] Failed to load module "../../amp-facebook/facebook-loader"
import {createLoaderLogo} from '../../amp-facebook/facebook-loader';
^

extensions/amp-facebook/0.1/amp-facebook.js:17: ERROR - [JSC_JS_MODULE_LOAD_WARNING] Failed to load module "../facebook-loader"
import {createLoaderLogo} from '../facebook-loader';
^

Addresses #23084 (comment)