Add a new subdirectory to allow wheels to bypass installation schemes

[qwcode]

Currently, the wheel format only supports installation to directories covered by the sysconfig schemes. This is a good thing for cross-platform wheel files, but can be an annoying limitation for platform specific ones.

The existing subdirectory containing additional content for sysconfig scheme paths is:
{distribution}-{version}.data/

A possible name for the new subdirectory would be:
{distribution}-{version}.custompaths/

All content listed under ".custompaths" would be installed directly to those paths, relative to the nominated root directory for the installation.

This would mostly be useful on *nix systems, as it allows things to be installed in FHS compliant locations under /etc, /var and elsewhere, without switching to platform specific packaging formats, and without a proliferation of FHS specific entries in the sysconfig scheme definitions.

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migrated from https://bitbucket.org/pypa/pypi-metadata-formats/issues/13/add-a-new-subdirectory-to-allow-wheels-to