When building an app, do not report remote package warnings

[agu-z]

We currently report warnings for all files we load. This means that the roc CLI will return a non-zero exit code when building an app that depends on a package with a warning. This can block the developer of the app from upgrading to a new version of the compiler, until the developer of the package fixes it.

When building an app, we should only report warnings that come from its source code or from packages that were loaded from a local file, instead of a URL.