Implement a method for marking a particular file as "not a plugin"

Requested Update

Description

The current methods for determining whether a file is a plugin or not are insufficient. We should implement a module-level attribute that specifically marks the containing module as NOT_A_PLUGIN. This should be inspected when identifying plugins and cause pluginify to skip those files while issuing a message to at the debug level indicating that a file was skipped.

At the same time, we should probably undo the changes that I made in #26 since they are a fragile workaround.

Background and Motivation

• [ OPTIONAL: Provide additional detailed information and background related to this issue ]

Alternative Solutions

• [ OPTIONAL: If appropriate, list alternative fixes that could be considered]

Environment

• [ IF APPROPRIATE: Corresponding repository versions ]

Code to demonstrate issue

• [ IF APPROPRIATE: Can include steps here to identify/demonstrate bug fix or desired feature ]

Checklist for Completion

• <!--- Include a list of steps that must be complete in order to close this Issue --->