Subfolders with the same name as a package prevent the package from being added

Having a subfolder with the same name as a package will silently prevent that package from being added to `requirements.txt`. For example, having a subfolder called `yaml` in a repository will prevent `PyYaml` from being added. This appears to be the issue in
#181 and #187.

One (partial) solution would be to check if the folder contains `__init__.py`, and if it does not, remove it from the `candidates` list in the [`get_all_imports()`](https://github.com/bndr/pipreqs/blob/060f52f59774c2fb8e24698da0a74cc7f6cfb8c9/pipreqs/pipreqs.py#L115) function , as it cannot (or at least shouldn't) be used as an import anyway. Another feature that could be helpful would be to check for items that are common to both `imports` and `candidates`, and then check if they map to a valid PyPi package, and then emit a warning that they are being suppressed.

I'm unsure if this could lead to unintended effects or break existing applications, but if this seems like a good idea I'm happy to implement it myself and submit a PR.

Start in pipreqs/pipreqs.py at get_all_imports(), linked from the issue, and trace how subfolders become entries in candidates and how imports are matched to packages. Reproduce the yaml/PyYaml case, then define and verify behavior for folders with and without __init__.py, including whether warnings are needed and whether existing applications remain unaffected.

Written by the indexing model from the issue text.