config/webpack/extensions.js
function getAliasesForExtensions( { extensionsDirectory } ) {
	const extensionsNames = fs
		.readdirSync( extensionsDirectory )
		.filter( filename => filename.indexOf( '.' ) === -1 ); // heuristic for finding directories

	const aliasesMap = {};
	extensionsNames.forEach(
		extensionName =>
			( aliasesMap[ extensionName ] = path.join( extensionsDirectory, extensionName ) )
	);
	return aliasesMap;
}
function getAliasesForExtensions( { extensionsDirectory } ) {
	const extensionsNames = fs
		.readdirSync( extensionsDirectory )
		.filter( filename => filename.indexOf( '.' ) === -1 ); // heuristic for finding directories

	const aliasesMap = {};
	extensionsNames.forEach(
		extensionName =>
			( aliasesMap[ extensionName ] = path.join( extensionsDirectory, extensionName ) )
	);
	return aliasesMap;
}
This function scans the /client/extensions directory in order to generate a map that looks like this:
{
sensei: 'absolute/path/to/wp-calypso/client/extensions/sensei',
woocommerce: 'absolute/path/to/wp-calypso/client/extensions/woocommerce',
....
}

Providing webpack with these aliases instead of telling it to scan client/extensions for every
module resolution speeds up builds significantly.
@returns {Object} a mapping of extension name to path