Chore: Add webpack5 support (part 1)

[ryan-roemer]

• Update fixtures to support webpack5 and use dummy webpack/webpack-cli root packages approach from webpack-stats-plugin
• Note: Does not actually enable webpack5 in tests for #140 as there's a lot more work to do there, so we're capturing the intermediate work here.

[ryan-roemer]

Note: Fails to build

diff --git a/package.json b/package.json
index 86ac75d..f48ba47 100644
--- a/package.json
+++ b/package.json
@@ -75,7 +75,7 @@
     "builder": "^5.0.0",
     "chai": "^4.2.0",
     "codecov": "^3.7.0",
-    "css-loader": "^3.5.3",
+    "css-loader": "^5.0.1",
     "deepmerge": "^4.2.2",
     "eslint": "^7.0.0",
     "eslint-config-formidable": "^4.0.0",
@@ -83,7 +83,7 @@
     "eslint-plugin-import": "^2.20.2",
     "eslint-plugin-promise": "^4.2.1",
     "execa": "^5.0.0",
-    "expose-loader": "^0.7.5",
+    "expose-loader": "^1.0.3",
     "inspectpack-test-fixtures": "file:test/fixtures",
     "mocha": "^8.2.1",
     "mock-fs": "^4.12.0",