`import runtime NonsenseModule` emits no diagnostics

Prerequisites

Please put an X between the brackets as you perform the following steps:

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

import runtime NonsenseModule succeeds even when NonsenseModule.lean does not exist

Context

I am investigating whether there is infrastructure to have dependencies which are only used at compile-time, and don't end up initialized in compiled binaries.

Steps to Reproduce

1. Create test.lean with import runtime NonsenseModule
2. Open it with VSCode
3. Build it with `lake build
4. Run lake env lean --deps test.lean

Expected behavior: Steps 2, 3, and 4 all fail because the module doesn't exist

Actual behavior: Only step 4 fails.

Versions

4.12.0

Additional Information

Related Zulip thread

Impact

Add 👍 to issues you consider important. If others are impacted by this issue, please ask them to add 👍 to it.

Start with the minimal test.lean containing import runtime NonsenseModule, then reproduce the behavior in VSCode, with lake build, and with lake env lean --deps test.lean. Trace how import runtime is handled across these paths; done means the missing module is diagnosed consistently by all three steps.