feature: Improve dependency-cycle error to show the cycle (#11519)

* Improve dependency-cycle error to show the cycle

Original message isn't help when diagnosing problems with a recipe...
Now it will print out all the X->Y dependencies that remain after the
irrelevant components have been eliminated.

Within that list will be the problematic cycle.

For example, as a test I introduced a cycle into my prototype VTK recipe,

I added:
self.cpp_info.components["vtksys"].requires.append("CommonDataModel")

The cycle would be:
vtksys -> CommonDataModel -> CommonCore -> vtksys

The message was a bit more verbose than that, but enough to be helpful:

ERROR: There is a dependency loop in 'self.cpp_info.components' requires:
   CommonTransforms requires CommonMath
   CommonDataModel requires CommonCore
   CommonMath requires CommonCore
   CommonCore requires vtksys
   CommonTransforms requires CommonCore
   CommonCore requires kwiml
   CommonDataModel requires CommonMath
   vtksys requires CommonDataModel
   CommonDataModel requires CommonTransforms
   CommonMath requires kissfft

* add test

Co-authored-by: memsharded <james@conan.io>

conans/model/build_info.py

@@ -608,8 +608,15 @@ def _get_sorted_components(self):
                             del components[comp_name]
                             break
                     else:
+                        dset = set()
+                        for comp_name, comp in components.items():
+                            for dep_name, dep in components.items():
+                                for require in self._filter_component_requires(dep.requires):
+                                    if require == comp_name:
+                                        dset.add("   {} requires {}".format(dep_name, comp_name))
+                        dep_mesg = "\n".join(dset)
                         raise ConanException("There is a dependency loop in "
-                                             "'self.cpp_info.components' requires")
+                                "'self.cpp_info.components' requires:\n{}".format(dep_mesg))
                 self._sorted_components = ordered
             else:  # If components do not have requirements, keep them in the same order
                 self._sorted_components = self._cpp_info.components

conans/test/integration/test_components.py

@@ -0,0 +1,25 @@
+import textwrap
+
+from conans.test.utils.tools import TestClient
+
+
+def test_components_cycles():
+    c = TestClient()
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+
+        class TestcycleConan(ConanFile):
+            name = "testcycle"
+            version = "1.0"
+
+            def package_info(self):
+                self.cpp_info.components["c"].requires = ["b"]
+                self.cpp_info.components["b"].requires = ["a"]
+                self.cpp_info.components["a"].requires = ["c"] # cycle!
+        """)
+    c.save({"conanfile.py": conanfile})
+    c.run("create .", assert_error=True)
+    assert "ERROR: There is a dependency loop in 'self.cpp_info.components' requires:" in c.out
+    assert "a requires c"
+    assert "b requires a"
+    assert "c rquires b"