jsii fails with bad error if outdir is under projectRoot

[rix0rrr]

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):

        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.