Remove additional repo_layer in migrations and snapshots path

[adamtharani]

There was a fix made in 0bfc8b8 to respect the priv from the repo for migrations and snapshots; however now when specifying priv: priv/repos/repo_a the snapshots are being put at priv/repos/repo_a/resource_snapshots/repo_a/extensions.json there is a redundant repo_a being added to the directory structure.

:priv - the directory where to keep repository data, like migrations, schema and more. Defaults to "priv/YOUR_REPO". It must always point to a subdirectory inside the priv directory