allow copying files into an already existing directory

[mattip]

Describe the bug A clear and concise description of what the bug is.

I would like to ship a build of OpenBLAS as a wheel. On posix, the build ships a libscipy-openblas.so which depends on libgfortran.so and libquadmath.so. The scipy-openblas shared object is the target bundled in the wheel under lib, then auditwheel/delocate is used to copy libgfortran and libquadmath into the lib directory. On auditwheel I can do this using --lib-sdir /lib. But in delocate, using delocate-wheel -L /lib raises an error here (note there is also a typo in the use of f-string, the f should be outside the quotes): https://github.com/matthew-brett/delocate/blob/eaba43dc531d0af348cb9b949a6c3f0790d207b2/delocate/delocating.py#L684-L687

To Reproduce Steps used to reproduce the behavior, such as the delocate commands used.

Expected behavior A clear and concise description of what you expected to happen.

Wheels used If a wheel is involved then consider attaching the original wheel (before being delocated) or linking to the repository where the original wheel can be created.

Platform (please complete the following information):

• OS version: [e.g. macOS 12, macOS 10.15]
• Delocate version: [e.g. 0.10.0]

Additional context Add any other context about the problem here.

What was the reasoning for wanting a clean directory?