Prevent copying PHP files from src/ into assets/

[delawski]

Summary

Fixes #6947

After introducing WordPress/gutenberg#38715, PHP files from the src folder are copied over to the output directory (i.e. assets). To prevent this, the CopyWebpackPlugin is excluded from the webpack shared plugins list.

It is safe to do so since the only other file type CopyWebpackPlugin copies is block.json that is not used in amp-wp.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[westonruter]

@delawski After merging this, removing assets/js/, and then re-running npm run dev, I'm still seeing the PHP files being copied:

	assets/js/Admin/
	assets/js/AmpSlugCustomizationWatcher.php
	assets/js/AmpWpPlugin.php
	assets/js/AmpWpPluginFactory.php
	assets/js/BackgroundTask/
	assets/js/Cli/
	assets/js/Component/
	assets/js/ConfigurationArgument.php
	assets/js/DependencySupport.php
	assets/js/DevTools/
	assets/js/Dom/
	assets/js/Editor/
	assets/js/Embed/
	assets/js/Exception/
	assets/js/ExtraThemeAndPluginHeaders.php
	assets/js/Icon.php
	assets/js/Infrastructure/
	assets/js/Instrumentation/
	assets/js/LoadingError.php
	assets/js/MobileRedirection.php
	assets/js/ObsoleteBlockAttributeRemover.php
	assets/js/Optimizer/
	assets/js/Option.php
	assets/js/OptionsRESTController.php
	assets/js/PairedRouting.php
	assets/js/PairedUrl.php
	assets/js/PairedUrlStructure.php
	assets/js/PairedUrlStructure/
	assets/js/PluginRegistry.php
	assets/js/PluginSuppression.php
	assets/js/QueryVar.php
	assets/js/ReaderThemeLoader.php
	assets/js/ReaderThemeSupportFeatures.php
	assets/js/RemoteRequest/
	assets/js/Sandboxing.php
	assets/js/Services.php
	assets/js/Support/
	assets/js/Validation/
	assets/js/ValidationExemption.php