feature: Improve dependency-cycle error to show the cycle

[paulharris]

Changelog: Feature: Improve dependency-cycle error to show the cycle.
Docs: Omit

---

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

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

[memsharded]

This seems a helpful addition of error message.
It would be necessary to add an integration test, please try to add it, or ask for help or guidance if you need it.

[paulharris]

@memsharded I had a look, and while I could probably figure it out with time, I'm a low on time.

I'd want to test that it outputs an error message with an expected error.
Recipe file:

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!

Error message (ignoring all the CCI hook errors I see):

conan create . 1.0@ccitest/cycle
...
ERROR: There is a dependency loop in 'self.cpp_info.components' requires:
   a requires c
   b requires a
   c requires b

How would I implement such a test?

[memsharded]

Thanks, that is enough, I'll add the test for you.

[memsharded]

Thanks very much for your contribution!
It will be released next 1.50