directives: add checked_by field to license(), add some license checks

Add a "checked_by" field to the `license()` directive so that we can track who verified
the license for a project. also check the license of 18 or so projects and mark them
checked.

lib/spack/spack/directives.py

@@ -912,14 +912,21 @@ def _execute_license(pkg, license_identifier: str, when):
 
 
 @directive("licenses")
-def license(license_identifier: str, when=None):
+def license(
+    license_identifier: str,
+    checked_by: Optional[Union[str, List[str]]] = None,
+    when: Optional[Union[str, bool]] = None,
+):
     """Add a new license directive, to specify the SPDX identifier the software is
     distributed under.
 
     Args:
-        license_identifiers: A list of SPDX identifiers specifying the licenses
-            the software is distributed under.
+        license_identifiers: SPDX identifier specifying the license(s) the software
+            is distributed under.
+        checked_by: string or list of strings indicating which github user checked the
+            license (if any).
         when: A spec specifying when the license applies.
+            when: A spec specifying when the license applies.
     """
 
     return lambda pkg: _execute_license(pkg, license_identifier, when)