Fix package's blocks assets override

My previous #1419 introduced a bug, causing some assets to not load when using custom templates.

This should fix that bug, and allow assets to be overrided correctly.

##### Available overrides:
`packages/{package}/blocks/{block}/view.css` -> 
`blocks/{block}/view.css`

`packages/{package}/blocks/{block}/css/*.css` -> 
`blocks/{block}/css/*.css`

##### Using Custom Templates (in package)
`packages/{package}/blocks/{block}/view.css` ->
`packages/{package}/blocks/{block}/templates/{$template}/view.css`

`packages/{package}/blocks/{block}/css/*.css` ->
`packages/{package}/blocks/{block}/templates/{$template}/css/*.css`

##### Using Custom Template (overriding existing)
`packages/{package}/blocks/{block}/templates/{$template}/view.css` ->
`blocks/{block}/templates/{template}/view.css`

`packages/{package}/blocks/{block}/templates/{$template}/css/*.css` ->
`blocks/{block}/templates/{template}/css/*.css`

##### Using Custom Template (added)
`packages/{package}/blocks/{block}/view.css` -> 
`blocks/{block}/templates/{added_template}/view.css`

`packages/{package}/blocks/{block}/css/*.css` -> 
`blocks/{block}/templates/{added_template}/css/*.css`

Same goes for js-assets.