jsii fails with bad error if outdir is under projectRoot

With the following setup:

```
# /path/to/projectDir
# package.json
{
"name": "teststatic",
"main": "index.js",
"types": "index.d.ts",
"jsii": {
"versionFormat": "full",
"targets": {
"dotnet": {
"namespace": "Bla.Module",
"packageId": "Bla.Module"
}
},
"tsc": {
"outDir": "dist"
}
},
}
```

So: we compile to an outdir `dist` *underneath* the project directory, if we then try to compile then `jsii` errors:

```
error JSII4: Could not find "main" file: /path/to/index.ts
```

It looks in the wrong directory for the entry point file.

The reason is this code (in JS form):

```js
const tscOutDir = (0, helpers_1.normalizeConfigPath)(projectInfo.projectRoot, program.getCompilerOptions().outDir);
if (tscOutDir != null) {
mainFile = path.relative(tscOutDir, mainFile);
// rootDir may be set explicitly or not. If not, inferRootDir replicates
// tsc's behavior of using the longest prefix of all built source files.
this.tscRootDir = program.getCompilerOptions().rootDir ?? inferRootDir(program);
if (this.tscRootDir != null) {
mainFile = path.join(this.tscRootDir, mainFile);
}
}
this.mainFile = path.resolve(projectInfo.projectRoot, mainFile);
```

Now `mainFile` contains `../index.ts` (outDir to mainFile), but it will be combined with `projectRoot` to come up with a file in the wrong directory.

What's worse, `jsii-config` recommended this setup with the `dist` directory to me.

Start with the package.json configuration shown and reproduce the failure using tsc.outDir set to dist beneath the project root. Trace the displayed mainFile path-resolution logic, then verify that compilation resolves the entry point to /path/to/projectDir/index.ts without JSII4.

Written by the indexing model from the issue text.