Allow overriding package's block's assets

Allow package's block's assets to be overrided.

Available overrides: 
`package\{pkgHandle}\blocks\{blockHandle}\view.js` -> `blocks\{blockHandle}\view.js`
`package\{pkgHandle}\blocks\{blockHandle}\view.css` -> `blocks\{blockHandle}\view.css`
`package\{pkgHandle}\blocks\{blockHandle}\js\*.js` -> `blocks\{blockHandle}\js\*.js`
`package\{pkgHandle}\blocks\{blockHandle}\css\*.css` -> `blocks\{blockHandle}\css\*.css`

 `ConcreteUrlsHelper::getBlockTypeAssetsURL` changed to return relative urls for block's assets