fix(SideEffectsFlag): Allow "false" and false

webpack#6074

Slightly modifies `SideEffectsFlagPlugin` to consider the string
`"false"` to be a valid input, whereas it previously only allowed for
the boolean `false`.

lib/optimize/SideEffectsFlagPlugin.js

@@ -16,7 +16,7 @@ class SideEffectsFlagPlugin {
 				const resolveData = data.resourceResolveData;
 				if(resolveData && resolveData.descriptionFileData && resolveData.relativePath) {
 					const sideEffects = resolveData.descriptionFileData.sideEffects;
-					const isSideEffectFree = sideEffects === false; // TODO allow more complex expressions
+					const isSideEffectFree = (sideEffects === false || sideEffects === "false");
 					if(isSideEffectFree) {
 						module.sideEffectFree = true;
 					}