Implement as_packages for forbidden contracts

Implement the funcitonality for treating the forbidden modules as a module rather than a package. If this is specified, we set the source_moduels and forbidden_modules as the package name rather than finding all downstream modules of that package.

docs/contract_types.rst

@@ -11,8 +11,9 @@ Forbidden modules
 
 Forbidden contracts check that one set of modules are not imported by another set of modules.
 
-Descendants of each module will be checked - so if ``mypackage.one`` is forbidden from importing ``mypackage.two``, then
-``mypackage.one.blue`` will be forbidden from importing ``mypackage.two.green``. Indirect imports will also be checked.
+By default, descendants of each module will be checked - so if ``mypackage.one`` is forbidden from importing ``mypackage.two``, then
+``mypackage.one.blue`` will be forbidden from importing ``mypackage.two.green``. Indirect imports will also be checked. This can be
+changed by setting add_packages to False: in that case, only explicitly listed modules will be checked, not their descendants.
 
 External packages may also be forbidden.
 
@@ -66,6 +67,9 @@ External packages may also be forbidden.
     - ``unmatched_ignore_imports_alerting``: See :ref:`Shared options`.
     - ``allow_indirect_imports``: If ``True``, allow indirect imports to forbidden modules without interpreting them
       as a reason to mark the contract broken. (Optional.)
+    - ``as_packages``: Whether to treat the source and forbidden modules as packages. If ``False``, each of the modules
+      passed in will be treated as a module rather than a package. Default behaviour is ``True`` (treat modules as
+      packages).
 
 Independence
 ------------

src/importlinter/contracts/forbidden.py

@@ -30,6 +30,10 @@ class ForbiddenContract(Contract):
         - unmatched_ignore_imports_alerting: Decides how to report when the expression in the
                              `ignore_imports` set is not found in the graph. Valid values are
                              "none", "warn", "error". Default value is "error".
+        - as_packages:       Whether to treat the source and forbidden modules as packages. If
+                             False, each of the modules passed in will be treated as a module
+                             rather than a package. Default behaviour is True (treat modules as
+                             packages).
     """
 
     type_name = "forbidden"
@@ -88,10 +92,14 @@ def sort_key(module):
                     }
 
                     if str(self.allow_indirect_imports).lower() == "true":
-                        chains = self._get_direct_chains(source_module, forbidden_module, graph, self.as_packages)
+                        chains = self._get_direct_chains(
+                            source_module, forbidden_module, graph, self.as_packages  # type:ignore
+                        )
                     else:
                         chains = graph.find_shortest_chains(
-                            importer=source_module.name, imported=forbidden_module.name
+                            importer=source_module.name,
+                            imported=forbidden_module.name,
+                            as_packages=self.as_packages,  # type:ignore
                         )
                     if chains:
                         is_kept = False
@@ -200,8 +208,16 @@ def _get_direct_chains(
         as_packages: bool,
     ) -> set[tuple[str, ...]]:
         chains: set[tuple[str, ...]] = set()
-        source_modules = self._get_all_modules_in_package(source_package, graph)
-        forbidden_modules = self._get_all_modules_in_package(forbidden_package, graph)
+        source_modules = (
+            self._get_all_modules_in_package(source_package, graph)
+            if as_packages
+            else {source_package}
+        )
+        forbidden_modules = (
+            self._get_all_modules_in_package(forbidden_package, graph)
+            if as_packages
+            else {forbidden_package}
+        )
         for source_module in source_modules:
             imported_module_names = graph.find_modules_directly_imported_by(source_module.name)
             for imported_module_name in imported_module_names: