[compiler] Don't emit spurious import { c as _c } for discarded functions (facebook#36500)

## What

Codegen registers `_c` (the memo cache import) as a side effect whenever
a function compiles with memo slots. The registration persists on
`ProgramContext.imports` even if the function is later discarded (`'use
no forget'`, `'use no memo'`, lint mode, validation errors). If other
applied functions in the file compile to 0 memo slots, the stale `import
{ c as _c } from "react/compiler-runtime";` leaks into the output.

## Fix

In `applyCompiledFunctions`, drop the memo cache import if no applied
function uses memo slots. If `react/compiler-runtime` has no remaining
specifiers, drop the module entry too so we don't emit a bare `import
"react/compiler-runtime";`.

## Reproducer

`use-no-forget-multiple-with-eslint-suppression.js`:

```js
import {useRef} from 'react';

const useControllableState = options => {};
function NoopComponent() {}

function Component() {
  'use no forget';
  const ref = useRef(null);
  // eslint-disable-next-line react-hooks/rules-of-hooks
  ref.current = 'bad';
  return <button ref={ref} />;
}
```

`NoopComponent` applies with 0 memo slots. `Component` is opted out, but
codegen already registered `_c` for it. Before:

```js
import { c as _c } from "react/compiler-runtime";
import { useRef } from "react";
```

After:

```js
import { useRef } from "react";
```

## Prior art

TS counterpart to the Rust port's fix in
7e26eb8. That commit also added
`no-cache-slots-no-import.js`, which codifies the "no memo slots, no
import" rule.

## Test plan

- `yarn snap`: 1719/1719 passing
- Snapshot for `use-no-forget-multiple-with-eslint-suppression` loses
its spurious `_c` import
- `no-cache-slots-no-import` still passes

Author: poteto

compiler/packages/babel-plugin-react-compiler/src/Entrypoint/Imports.ts

@@ -151,6 +151,17 @@ export class ProgramContext {
     );
   }
 
+  removeMemoCacheImport(): void {
+    const moduleImports = this.imports.get(this.reactRuntimeModule);
+    if (moduleImports == null) {
+      return;
+    }
+    moduleImports.delete('c');
+    if (moduleImports.size === 0) {
+      this.imports.delete(this.reactRuntimeModule);
+    }
+  }
+
   /**
    *
    * @param externalFunction

compiler/packages/babel-plugin-react-compiler/src/Entrypoint/Program.ts

@@ -775,6 +775,13 @@ function applyCompiledFunctions(
 
   // Forget compiled the component, we need to update existing imports of useMemoCache
   if (compiledFns.length > 0) {
+    // Codegen may have registered `_c` for a function that was later discarded.
+    const anyAppliedUsesMemo = compiledFns.some(
+      result => result.compiledFn.memoSlotsUsed > 0,
+    );
+    if (!anyAppliedUsesMemo) {
+      programContext.removeMemoCacheImport();
+    }
     addImportsToProgram(program, programContext);
   }
 }

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/use-no-forget-multiple-with-eslint-suppression.expect.md

@@ -25,7 +25,6 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
 import { useRef } from "react";
 
 const useControllableState = (options) => {};