Deprecate 'cppflags' in favor of 'cxxflags' in class CppInfo

[jgsogo]

Changelog: Fix: Deprecate 'cppflags' in favor of 'cxxflags' in class CppInfo
Docs: conan-io/docs#1091

closes #4337

• Update docs

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In order to remove all the usages of the deprecated member in our codebase, I find one problem: the files written by the generators...

For example, the conanbuildinfo.txt file now contains

[cppflags_Hello]
FLAG

and the expected (for Conan v2.0) should be:

[cxxflags_Hello]
FLAG

During the transition, we can output something like the following one, to keep compatibility:

[cppflags_Hello]
FLAG
[cxxflags_Hello]
FLAG

So, our reader should be able to deal with the first option and the last one until Conan V2 (checking that both sections contain the same information). This is easy for the txt generator, but it might require some more testing for other generators.

Maybe we can merge this PR jsut with the deprecation notice as the issue states, and then we can open a different PR to fix all the usages of this deprecated code. WDYT?

[memsharded]

Looks good, the only thing if we are going to remove it in Conan 2.0, we might want to raise an Exception, so defining it is not silently dismissed.

[danimtb]

I think the approach proposed is right and the [cxxflags] could be introduced in another PR in the future as there are other generators with the same problem.

Please make sure to update the docs!

[jgsogo]

@memsharded

Looks good, the only thing if we are going to remove it in Conan 2.0, we might want to raise an Exception, so defining it is not silently dismissed.

I would like to use the approach recommended by the deprecation package we are using, but I cannot get conans.__version__ in the build_info.py file (circular dependency). I can write a test on my own, but it is too much code:

    """
    TODO: Need to solve the circular dependency when importing `conans.__version__` in build_info.py
    @deprecation.fail_if_not_remove
    def test_deprecation_v2(self):
         cpp_info = CppInfo("roothpath")
         self.assertListEqual(cpp_info.cppflags, [])
    """

    def test_deprecation_v2(self):
        cpp_info = CppInfo("roothpath")

        import warnings
        from conans import __version__
        from packaging import version

        if version.parse(__version__) >= version.parse("2.0"):
            with warnings.catch_warnings(record=True) as caught_warnings:
                warnings.simplefilter("always")

                self.assertListEqual(cpp_info.cppflags, [])

                if caught_warnings:
                    self.fail("CppInfo::cppflags must be removed in Conan 2.0")

Other option is to move conans.__version__ from conans/__init__.py to conans/version.py to avoid the circular dependency, and use the commented version of the test

[lasote]

I'm not worried about raising an error in Conan 2.0, we will manage. Please add the docs to merge it.

[jgsogo]

The idea is to implement a test that fails in Conan v2.0 (so it will remember us that we have to delete that deprecated functionality), but the code will keep working the same...

... on my way to docs!

[danimtb]

Docs are ready!