Deprecating usage of directories in export_file.

Created by: styurin

Exporting directories gives us less control over what is used as an input of a rule and can potentially introduce rule key divergence. The risk of encountering this problem is higher in local development (some editors can leave temporary or backup files next to the original files) and also with some tools that use temporary files (like code generators.)

The general recommendation is to use glob to declare inputs from a directory, zip_file to pack multiple files in one file or genrule without a directory as an output.

3a3f7e06 introduced an option to disable support. This option can be used to find all violations and refactor them.

We plan to remove this functionality in future versions.