TSC should not error on valid subpath specifier syntax (TS2877)

### 🔎 Search Terms

TS2877

### 🕗 Version & Regression Information

This changed between versions 5.6 and 5.7 (best guess; 5.7 introduced `rewriteRelativeImportExtensions`).

### ⏯ Playground Link

https://www.typescriptlang.org/play/?rewriteRelativeImportExtensions=true&resolvePackageJsonExports=true&resolvePackageJsonImports=true&noUncheckedSideEffectImports=true&allowArbitraryExtensions=true&allowImportingTsExtensions=true#code/PTAEEkFsAcHsCcAuBnUAbAlgawKakQBY7J4DGAhgK4mgAqAygEwAcA7KwDQBQIoA7kQB2oDMIBm5UonxF4OAOSpBsQqIDm-eLEEbyAI1iVphHJAB0XHmAACKALQ4AHtBxSH8LfABcMjKj6GaAAmoHIAjpQYcqDkoNCSWORqOGYAVsja+LChxLBoAG44XBgwCNIARADE5JikKSjlANxcQA

### 💻 Code

```ts
// Imports like these cause TS2877,
// when in fact there's nothing wrong about them.
import "#alice.ts";
```

### 🙁 Actual behavior

```txt
error TS2877: This import uses a '.ts' extension to resolve to an input TypeScript file, but will not be rewritten during emit because it is not a relative path.
```

### 🙂 Expected behavior

Don't error.

### Additional information about the issue

Specifiers aren't paths. Especially subpath imports and exports allow for almost arbitrary string rewriting. The compiler is blatantly wrong about this.

Edit: I've created a [repro repo](https://github.com/valler/repro-repo-for-ms-typescript-issue-63129).

Start with the linked Playground reproduction and the repro repository, using the listed compiler options and the TS2877 diagnostic as entry points. Trace why the '#alice.ts' subpath import is treated as a non-rewritable input file. Done means this valid specifier no longer produces TS2877 while the reported relative-path behavior remains intact.

Written by the indexing model from the issue text.