raise ConanException if folder creation path is an existing file

[davidsanfal]

Changelog: Fix: Raise an exception when trying to create a directory at a path where a file of the same name already exists.
Docs: Omit

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[memsharded]

I know this check is super fast, but it still reads a bit overly protective, "overkill":

• It protects against an error that is extremely unlikely
• It only provides a better error message (or avoid a stack trace)
• These low-level functions might be called many times inside Conan internal code. Maybe not the case for this one, but still

It might be better to just change os.path.exists() => os.path.isdir() and let the subsequent os.makedirs() to raise the exception about that?

These functions are also a bit legacy, in the sense that they were introduced before the exists_ok=True argument was introduced in os.makedirs(). New code is already using directly os.makedirs() instead, so this wouldn't protect against this error in those cases.

[AbrilRBS]

Note that the code here raises after the fact with a stacktrace when trying to build the generators, that's why the idea to prettify the root cause came to be, but yeah maybe it ends up being a tad too defensive

[davidsanfal]

The problem arises when there is a file, so isdir leaves us in the same situation. It will try to create a path that exists and will fail with an uncontrolled error trace.
We can add the exists_ok=True flag, but in this case, I don't think it's better to indicate that the folder cannot be created. It is now more a question of UI and expected result.

[memsharded]

Maybe the approach is to protect the caller of the mkdir, capturing the exception there.

The working hypothesis for mkdir, or precondition is that such a case doesn't exist. Most of the time, the mkdir might be used to create dirs inside the Conan cache, which is 100% controlled that such spureous files won't exist.

In any case, I was being also extremely picky about this, I think this is very likely very fast to execute, without any performance penalty anywhere, it was mostly to raise a general design principles awareness and potential risks of doing this kind of things if something is in a critical or massively called path.

This can be merged as-is, no need to invest more time in it, thanks!

[memsharded]

After talking with @AbrilRBS, I think this is still a bit too defensive.
Lets go to protect only the "build" folder-file collision in:

def run_build_method(conanfile, hook_manager):
    if os.path.isfile(conanfile.build_folder):
           raise ConanException(f"Can't create the build folder, it is an existing file: {conanfile.build_folder}")
    mkdir(conanfile.build_folder)
    ...