(#11627) linter - check ConanFile import and cross_building

* linter - check ConanFile import and cross_building

* remove unused code

* Add docs

* improve docs

* example - abseil

* md syntax

* revert uneeded changes

* Update docs/v2_linter.md

Co-authored-by: Daniel <danimanzaneque@gmail.com>

Co-authored-by: Daniel <danimanzaneque@gmail.com>

linter/check_import_conanfile.py

@@ -0,0 +1,28 @@
+
+from pylint.checkers import BaseChecker
+from pylint.interfaces import IAstroidChecker
+from astroid import nodes, Const, AssignName
+
+
+class ImportConanFile(BaseChecker):
+    """
+       Import ConanFile from new 'conan' module
+    """
+
+    __implements__ = IAstroidChecker
+
+    name = "conan-import-conanfile"
+    msgs = {
+        "W9006": (
+            "Import ConanFile from new module: `from conan import ConanFile`. Old import is deprecated in Conan v2.",
+            "conan-import-conanfile",
+            "Import ConanFile from new module: `from conan import ConanFile`. Old import is deprecated in Conan v2.",
+        ),
+    }
+
+    def visit_importfrom(self, node: nodes.ImportFrom) -> None:
+        basename = node.modname
+        if basename == 'conans':
+            names = [name for name, _ in node.names]
+            if 'ConanFile' in names:
+                self.add_message("conan-import-conanfile", node=node)