Try to derive default file mode from input types

The current mechanism for setting file modes in `pkg_files` is ad-hoc and requires human intervention to set permissions, even when it's relatively obvious what they're supposed to be: runnable binaries should be runnable, libraries shouldn't.

Investigate mechanisms for determining how we might derive this information. The most likely source of this information is the provider interfaces.

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That's a good point, however I don't think we're quite there. We'd have to do some introspection magic in `pkg_files` to identify the nature of the dependent rules. Providers? Rule names? This will need to be investigated. At first brush, this feels like it'll be hard to get right.

I feel that this can be postponed to a later task. I'll make a ticket for it.

_Originally posted by @nacl in https://github.com/bazelbuild/rules_pkg/pull/290#discussion_r582332072_