Fix for multi-line use calls

[katafrakt]

When I tried to plug #333 into Expert, I found it not working for some Ecto definitions. At first I thought this is due to transitive use of use (like use AshPostgres.Repo which calls use Ecto.Repo inside its __using__), but ultimately I found this is because of multi-line uses, like AshPostgres does:

        use Ecto.Repo,
          adapter: opts[:adapter] || Ecto.Adapters.Postgres,
          otp_app: otp_app

When we were calling use_site?/3 on the first line, it tried to do Code.string_to_quoted on a line ending with a comma, returning {:error, _} and not finding it as a use site. This PR replaces the check with a regex-based. I was trying to find some false positives it can generate, but I don't think there are some. The obvious one is about comments, but I don't think a line with a comment can be a param to use_site? currently.

The commit history is a mess here of all my previous attempt, so it needs squashing.

[katafrakt]

@lukaszsamson do you think it's ok to merge? I would love to pull these new changes to Expert, but I think this fix is necessary for that.