gopages: Refuse to delete output directory unless it resembles a gopages result

[JohnStarich]

Refuse to delete output directory unless it resembles a gopages result.

Now gopages only erases the output directory if:

• The directory does not exist
• OR the directory is empty
• OR the lib, pkg, src directories, and index.html file are present

Fixes https://github.com/JohnStarich/go/issues/27

[github-actions[bot]]

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