bdist gives cant copy error for package_data file which exists

Originally reported by: rogermarsh (Bitbucket: rogermarsh, GitHub: Unknown)

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python setup.py bdist gives:

error: can't copy 'icence.txt': doesn't exist or not a regular file

where current directory is swiggedapi and licence.txt exists in this directory.

The package related arguments to setup() are:

    package_dir={'swiggedapi':''},
    packages=['swiggedapi'],
    package_data={'': ['licence.txt', '_swigged.pyd']},

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/341

Original comment by rogermarsh (Bitbucket: rogermarsh, GitHub: Unknown):

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Sample project attached

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Prefer relpath to string slicing for computing a path relative to a base. Fixes #341.

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Although I have included a patch to avoid the off-by-one error (truncation of the first character of the filenames), the issue was encountered by the empty string in the key of package_data. According to the docs, "the value must be a mapping from package name to a list of relative path names that should be copied into the package." I suspect that the empty string isn't a valid package name.