refactor: Index requirements by when spec

Part 3 of reworking all package metadata to key by `when` conditions.

Changes conflict dictionary structure from this:

    { (requirement_spec, ...): [(when_spec, policy, msg)] }

to this:

    { when_spec: [((requirement_spec, ...), policy, msg), ...] }

lib/spack/spack/audit.py

@@ -967,10 +967,10 @@ def _extracts_errors(triggers, summary):
             summary = f"{pkg_name}: wrong 'when=' condition for the '{provided}' virtual"
             errors.extend(_extracts_errors(triggers, summary))
 
-        for _, triggers in pkg_cls.requirements.items():
+        for when, requirements, details in _error_items(pkg_cls.requirements):
-            triggers = [when_spec for when_spec, _, _ in triggers]
+            errors.append(
-            summary = f"{pkg_name}: wrong 'when=' condition in 'requires' directive"
+                error_cls(f"{pkg_name}: wrong 'when=' condition in 'requires' directive", details)
-            errors.extend(_extracts_errors(triggers, summary))
+            )
 
         for when, _, details in _error_items(pkg_cls.patches):
             errors.append(

lib/spack/spack/directives.py

@@ -965,7 +965,7 @@ def license(
 
 
 @directive("requirements")
-def requires(*requirement_specs, policy="one_of", when=None, msg=None):
+def requires(*requirement_specs: str, policy="one_of", when=None, msg=None):
     """Allows a package to request a configuration to be present in all valid solutions.
 
     For instance, a package that is known to compile only with GCC can declare:
@@ -984,7 +984,7 @@ def requires(*requirement_specs, policy="one_of", when=None, msg=None):
         msg: optional user defined message
     """
 
-    def _execute_requires(pkg):
+    def _execute_requires(pkg: "spack.package_base.PackageBase"):
         if policy not in ("one_of", "any_of"):
             err_msg = (
                 f"the 'policy' argument of the 'requires' directive in {pkg.name} is set "
@@ -997,9 +997,10 @@ def _execute_requires(pkg):
             return
 
         # Save in a list the requirements and the associated custom messages
-        when_spec_list = pkg.requirements.setdefault(tuple(requirement_specs), [])
+        requirement_list = pkg.requirements.setdefault(when_spec, [])
         msg_with_name = f"{pkg.name}: {msg}" if msg is not None else msg
-        when_spec_list.append((when_spec, policy, msg_with_name))
+        requirements = tuple(spack.spec.Spec(s) for s in requirement_specs)
+        requirement_list.append((requirements, policy, msg_with_name))
 
     return _execute_requires
 

lib/spack/spack/package_base.py

@@ -562,6 +562,9 @@ class PackageBase(WindowsRPath, PackageViewMixin, metaclass=PackageMeta):
     versions: dict
     dependencies: Dict["spack.spec.Spec", Dict[str, "spack.dependency.Dependency"]]
     conflicts: Dict["spack.spec.Spec", List[Tuple["spack.spec.Spec", Optional[str]]]]
+    requirements: Dict[
+        "spack.spec.Spec", List[Tuple[Tuple["spack.spec.Spec", ...], str, Optional[str]]]
+    ]
     patches: Dict["spack.spec.Spec", List["spack.patch.Patch"]]
 
     #: By default, packages are not virtual

lib/spack/spack/solver/asp.py

@@ -1303,8 +1303,8 @@ def package_requirement_rules(self, pkg):
 
     def requirement_rules_from_package_py(self, pkg):
         rules = []
-        for requirements, conditions in pkg.requirements.items():
+        for when_spec, requirement_list in pkg.requirements.items():
-            for when_spec, policy, message in conditions:
+            for requirements, policy, message in requirement_list:
                 rules.append(
                     RequirementRule(
                         pkg_name=pkg.name,