Allow opt-out of find_files behavior. (issue 561)

[daveware-nv]

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~Changes it so find_files is only enabled with config parameter enable_find_files is set to true, otherwise will always return empty list.~

Adds config argument enable_find_files that when set to false allows find_files behavior to be disabled.

Addresses https://github.com/pypa/setuptools_scm/issues/561

[daveware-nv]

OK, I've switched the default to true.

[ljames8]

isn't this PR good to be merged now the requested changes have been made ?

[RonnyPfannschmidt]

cant merge this yet as opting out of the file finders possibly opts into version finding + there is a bug

[Lenbok]

@RonnyPfannschmidt Do you have more information, e.g. what is the bug that is preventing this being merged?

[webknjaz]

@daveware-nv looks like the PR title needs to be fixed to reflect the post-review changes. Could you update it?

[RonnyPfannschmidt]

currently there are many merge conflicts - im unable to merge as is

in the next major release i want to switch from file finders to custom setuptools subcommands as currently file finder opt out requires setuptools_scm "opt-in"

[jaraco]

cant merge this yet as opting out of the file finders possibly opts into version finding + there is a bug

In https://github.com/pypa/setuptools_scm/pull/364#discussion_r349931908, I advocated for there being a separate boolean to enable (or disable) version inference. Probably that needs to be implemented (first) so people can opt out of versions inference as well. Either that, or the tool.setuptools_scm needs to be deprecated and superseded by a new name that could have different defaults (and doesn't use the section header to imply enablement of a feature).