Add relationship between issuer and module to traces

Sets the parent span for module traces to the module that caused this current module to compile, this makes tracing why a module was compiled much simpler. Also added `NEXT_WEBPACK_PARALLELISM` to allow for better traces of individual modules, this should only be used in combination with tracing.

packages/next/build/webpack-config.ts

@@ -860,6 +860,7 @@ export default async function getBaseWebpackConfig(
   const emacsLockfilePattern = '**/.#*'
 
   let webpackConfig: webpack.Configuration = {
+    parallelism: Number(process.env.NEXT_WEBPACK_PARALLELISM) || undefined,
     externals: !isServer
       ? // make sure importing "next" is handled gracefully for client
         // bundles in case a user imported types and it wasn't removed

packages/next/build/webpack/plugins/profiling-plugin.ts

@@ -101,9 +101,11 @@ export class ProfilingPlugin {
           return module.userRequest.split('.').pop()
         })()
 
+        const issuerModule = compilation.moduleGraph.getIssuer(module)
+
         const span = trace(
           `build-module${moduleType ? `-${moduleType}` : ''}`,
-          compilerSpan.id
+          issuerModule ? spans.get(issuerModule)?.id : compilerSpan.id
         )
         span.setAttribute('name', module.userRequest)
         spans.set(module, span)