[PP-1] Skip parsing *Base files in plugin folders during attribute plugin discovery

Problem/Motivation

Plugin discovery registers an entry in the file cache for every file in a plugin folder, even if the entry is essentially NULL because the file does not contain a plugin definition. Base classes do not contain plugin definitions, so they should be skipped as discovery iterates through the directories, so that they do not have entries in the file cache. Once coding standards are in place that base class names must (and only base class names can) end in "Base", these files can be skipped.

This is blocked on #1567920: Naming standard for abstract/base classes.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

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