Only load files in workspace for --check (skip libraries)

[emmericp]

The current behavior of --check is to run diagnostics on all files that are loaded including defined libraries. I believe checking libraries is undesirable because:

1. you might not own the code in there and any warning triggered in there can be non-actionable for you
2. libraries can be very large and make check take a long time. For our project it adds about 2 minutes of time to check libraries and get warnings we can't do anything about anyways

Please let me know if you would prefer this behavior to be customizable instead, but I believe the current behavior is just undesirable in general.

[sumneko]

Thank you!