Validation broken when loading split models via `-t`

[rahearn]

Describe the bug

Validation returns incorrect failures when called with the -t flag, at least for -t system-security-plan when the model has been split using trestle split

To Reproduce

Steps to reproduce the behavior:

1. Create a new system-security-plan trestle create -t system-security-plan -o validate-test
2. Edit ssp to put a valid profile href in
3. Verify that trestle validate -f system-security-plans/validate-test/system-security-plan.json and trestle validate -t system-security-plan -n validate-test correctly report that the model is valid
4. Split the model: trestle split -f system-security-plans/validate-test/system-security-plan.json -e "system-security-plan.metadata"
5. Verify that trestle validate -f system-security-plans/validate-test/system-security-plan.json still reports that the model is valid
6. Try to validate with trestle validate -t system-security-plan -n validate-test

Expected behavior

The model is still valid

Actual behavior

Step 6 reports that the model has extra fields.

Screenshots / Logs.

Environment

• OS: docker
• Python version: 3.11
• Installed packages: compliance-trestle 3.4.0