Bring validation from legacy config to new graph-based config builder

[austinvalle]

TODO: This PR still needs some more tests and maybe some more validation to ensure we didn't lose anything else 👀 . Also a changelog 😅

This fixes a bug where child module validation doesn't run, as we didn't port the config validation over when we added the new graph-based module installer. This should now accurately detect invalid child module:

• Imports
• Backend / cloud blocks
• List resources

This change needs to be applied to main and to v1.16 as well, although the child import validation should be omitted as we offically support that now!

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[github-actions]

Changelog Warning

Currently this PR would target a v1.15 release. Please add a changelog entry for in the .changes/v1.15 folder, or discuss which release you'd like to target with your reviewer. If you believe this change does not need a changelog entry, please add the 'no-changelog-needed' label.