[snap] Make watch mode go fast(er)

While optimizing per @josephsavona's suggestions in #1592, I noticed that we 
were clearing quite a few require cache entries. 

As of this PR, `Object.keys(require.cache)` holds 

- 1258 entries total 

- 67 files compiled from Forget source code (this is what 
`ts.createWatchCompilerHost` modifies) 

- 1120 babel source files (from node_modules) 

When working on watch mode, I'm almost always making changes to Forget source or 
test fixture files. It's a bit faster to just clear those entries (assuming that 
babel has no global state we need to invalidate). 

On my computer, re-running tests in watch mode (triggered by source code 
changes) takes: 

|   |  All tests  | One test (filter) | 

|-- |--------|----------| 

| current | 4.7s  | 1.8s  | 

| this PR  |  1.8s  | 0.1s  |

compiler/forget/packages/snap/src/compiler-worker.ts

@@ -11,24 +11,14 @@ import { exists } from "./utils";
 
 const originalConsoleError = console.error;
 
-// Subpaths to ignore when clearing the require cache
-const ignoredRequireSubpaths: Array<string> = [
-  // compiler worker runner files
-  "node_modules/jest-worker",
-  // snap source files
-  "packages/snap",
-];
-const ignoredRequirePaths: Set<string> = new Set(
-  Object.keys(require.cache).filter(
-    (path) =>
-      !ignoredRequireSubpaths.every((ignored) => !path.includes(ignored))
-  )
-);
-
+// Try to avoid clearing the entire require cache, which (as of this PR)
+// contains ~1250 files. This assumes that no dependencies have global caches
+// that may need to be invalidated across Forget reloads.
+const invalidationSubpath = "react-forget/forget/dist";
 let version: number | null = null;
 export function clearRequireCache() {
   Object.keys(require.cache).forEach(function (path) {
-    if (!ignoredRequirePaths.has(path)) {
+    if (path.includes(invalidationSubpath)) {
       delete require.cache[path];
     }
   });

compiler/forget/packages/snap/src/runner.ts

@@ -441,13 +441,14 @@ export async function main(opts: RunnerOptions): Promise<void> {
         const start = performance.now();
         clearConsole();
         console.log("Running tests...");
+        // we don't clear console after this point, since
+        // it may contain debug console logging
         const results = await run(
           worker,
           opts,
           filterMode ? testFilter : null,
           compilerVersion
         );
-        clearConsole();
         if (mode === Mode.Update) {
           update(results);
         } else {