No warning produced for import element with configure-commands set

Summary

When building and element with kind: import and...

config:
  configure-commands:
  - commandA.sh

BuildStream goes about the build without warning or failing. This is perhaps confusing for the user expecting their commands to be executed.

Steps to reproduce

Build this element within a project

kind: import

sources:
- kind: git
  url: https://gitlab.com/BuildStream/buildstream.git
  track: master

config:
  configure-commands:
  - ls

What is the current bug behavior?

BuildStream performs the build with no warnings or indication that anything is wrong.

What is the expected correct behavior?

BuildStream should at least warn the user that they're probably not going to see the behaviour they're expecting.

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: master

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