fix: don't require webpack dependency at runtime (#492)

lib/core/hooks.js

@@ -1,6 +1,5 @@
 import { resolve, posix } from 'path'
 import merge from 'lodash.mergewith'
-import { DefinePlugin } from 'webpack'
 import * as Sentry from '@sentry/node'
 import { canInitialize, clientSentryEnabled, envToBool, serverSentryEnabled } from './utils'
 import { resolveRelease, resolveClientOptions, resolveServerOptions } from './options'
@@ -46,9 +45,10 @@ export async function buildHook (moduleContainer, moduleOptions, logger) {
 
   // Tree shake debugging code if not running in dev mode and Sentry debug option is not enabled on the client.
   if (!clientOptions.dev && !clientOptions.config.debug) {
+    const webpack = await import('webpack').then(m => m.default || m)
     moduleContainer.options.build.plugins = moduleContainer.options.build.plugins || []
     moduleContainer.options.build.plugins.push(
-      new DefinePlugin({
+      new webpack.DefinePlugin({
         __SENTRY_DEBUG__: 'false',
       }),
     )