Test package detection in a systematic way (#18175)

This PR adds a new audit sub-command to check that detection of relevant packages
is performed correctly in a few scenarios mocking real use-cases. The data for each 
package being tested is in a YAML file called detection_test.yaml alongside the 
corresponding package.py file.

This is to allow encoding detection tests for compilers and other widely used tools, 
in preparation for compilers as dependencies.

.github/workflows/audit.yaml

@@ -34,6 +34,7 @@ jobs:
       run: |
           . share/spack/setup-env.sh
           coverage run $(which spack) audit packages
+          coverage run $(which spack) audit externals        
           coverage combine
           coverage xml
     - name: Package audits (without coverage)
@@ -41,6 +42,7 @@ jobs:
       run: |
           . share/spack/setup-env.sh
           $(which spack) audit packages
+          $(which spack) audit externals
     - uses: codecov/codecov-action@eaaf4bedf32dbdc6b720b63067d99c4d77d6047d # @v2.1.0
       if: ${{ inputs.with_coverage == 'true' }}
       with: