Add Import Linter contract

It's about time!

Author: seddonym

.importlinter

@@ -0,0 +1,22 @@
+[importlinter]
+root_package = grimp
+
+[importlinter:contract:1]
+name=Layered architecture
+type=layers
+containers =
+    grimp
+layers=
+    main
+    adaptors
+    application
+    domain
+    exceptions
+exhaustive = true
+ignore_imports =
+    ; Grimp doesn't understand extension modules: it thinks grimp._rustgrimp
+    ; is an object within grimp, rather than a module in its own right,
+    ; so we ignore these imports here.
+    grimp.adaptors.graph -> grimp
+    grimp.adaptors.importscanner -> grimp
+    grimp.adaptors.filesystem -> grimp

tox.ini

@@ -51,6 +51,7 @@ basepython = py313
 deps =
     black==23.9.1
     flake8==7.1.1
+    import-linter==2.3
     mypy==1.11.2
     pytest-stub==1.1.0
     types-pyyaml==6.0.12.20240917
@@ -59,6 +60,7 @@ commands =
     black --check src tests
     flake8 src tests
     mypy src/grimp tests
+    lint-imports
 
 [testenv:benchmark]
 basepython = py313