[fs-detectors] Add fix for if monorepo and proj at root (#9353)

If there is a monorepo that has one project at the root level we want to
return commands without the relative to root prefixes

---------

Co-authored-by: Chris Barber <chris.barber@vercel.com>

packages/fs-detectors/src/monorepos/get-monorepo-default-settings.ts

@@ -21,12 +21,19 @@ export class MissingBuildTarget extends Error {
   }
 }
 
+type MonorepoDefaultSettings = {
+  buildCommand?: string | null;
+  installCommand?: string | null;
+  commandForIgnoringBuildStep?: string;
+  monorepoManager: string;
+} | null;
+
 export async function getMonorepoDefaultSettings(
   projectName: string,
   projectPath: string,
   relativeToRoot: string,
   detectorFilesystem: DetectorFilesystem
-) {
+): Promise<MonorepoDefaultSettings> {
   const [monorepoManager, packageManager] = await Promise.all([
     detectFramework({
       fs: detectorFilesystem,
@@ -64,14 +71,27 @@ export async function getMonorepoDefaultSettings(
       throw new MissingBuildPipeline();
     }
 
+    if (projectPath === '/') {
+      return {
+        monorepoManager: 'turbo',
+        buildCommand: 'npx turbo run build',
+        installCommand: packageManager ? `${packageManager} install` : null,
+        commandForIgnoringBuildStep: 'npx turbo-ignore',
+      };
+    }
+
     return {
       monorepoManager: 'turbo',
-      buildCommand: `cd ${relativeToRoot} && npx turbo run build --filter={${projectPath}}...`,
+      buildCommand: projectPath
+        ? `cd ${relativeToRoot} && npx turbo run build --filter={${projectPath}}...`
+        : null,
       installCommand:
         packageManager === 'npm'
           ? `${packageManager} install --prefix=${relativeToRoot}`
-          : `${packageManager} install`,
-      commandForIgnoringBuildStep: `npx turbo-ignore`,
+          : packageManager
+          ? `${packageManager} install`
+          : null,
+      commandForIgnoringBuildStep: 'npx turbo-ignore',
     };
   } else if (monorepoManager === 'nx') {
     // No ENOENT handling required here since conditional wouldn't be `true` unless `nx.json` was found.
@@ -111,13 +131,24 @@ export async function getMonorepoDefaultSettings(
       }
     }
 
+    if (projectPath === '/') {
+      return {
+        monorepoManager: 'nx',
+        buildCommand: 'npx nx build',
+        installCommand: packageManager ? `${packageManager} install` : null,
+      };
+    }
     return {
       monorepoManager: 'nx',
-      buildCommand: `cd ${relativeToRoot} && npx nx build ${projectName}`,
+      buildCommand: projectName
+        ? `cd ${relativeToRoot} && npx nx build ${projectName}`
+        : null,
       installCommand:
         packageManager === 'npm'
           ? `${packageManager} install --prefix=${relativeToRoot}`
-          : `${packageManager} install`,
+          : packageManager
+          ? `${packageManager} install`
+          : null,
     };
   }
   // TODO (@Ethan-Arrowood) - Revisit rush support when we can test it better