'debug' should be an explicit dependency

[Dave Honneffer]

It seems (unless I've not looked in the right place) that debug is being used, but it isn't specified in the package dependencies.

It likely still works since several dependencies list it as a sub-dependency, but this means its version can't be controlled for.

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[Xin Du (Clark)]

@pearofducks Thanks for the notice, I have removed debug in ac070a1 since it's not very necessary.