Symlink after running npm ci/rebuild not before

With node 16 and the npm7 that comes with it, it was noticed that it
refuses to follow a node_modules symlink if it was created prior to the
run of npm commands. So we should run the commands first, then relocate
the modules dir to the layer and then point the symlink there from the
working directory of the app.

In #290, this was dealt with for the "npm install" command. This commit
applies the same logic to rebuild and ci commands. See matrix here for
how the buildpack chooses commands.
https://paketo.io/docs/reference/nodejs-reference

The only important changes are in ci_build_process.go and
rebuild_build_process.go. Others are testdata changes.

Fixes: paketo-buildpacks/nodejs#492

Author: arjun024

ci_build_process.go

@@ -2,6 +2,7 @@ package npminstall
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -61,16 +62,6 @@ func (r CIBuildProcess) Run(modulesDir, cacheDir, workingDir string) error {
 		return err
 	}
 
-	err = fs.Move(filepath.Join(workingDir, "node_modules"), filepath.Join(modulesDir, "node_modules"))
-	if err != nil {
-		return err
-	}
-
-	err = os.Symlink(filepath.Join(modulesDir, "node_modules"), filepath.Join(workingDir, "node_modules"))
-	if err != nil {
-		return err
-	}
-
 	buffer := bytes.NewBuffer(nil)
 	args := []string{"ci", "--unsafe-perm", "--cache", cacheDir}
 
@@ -90,5 +81,24 @@ func (r CIBuildProcess) Run(modulesDir, cacheDir, workingDir string) error {
 		return fmt.Errorf("npm ci failed: %w", err)
 	}
 
+	_, err = os.Stat(filepath.Join(workingDir, "node_modules"))
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		} else {
+			return fmt.Errorf("unable to stat node_modules in working directory: %w", err)
+		}
+	}
+
+	err = fs.Move(filepath.Join(workingDir, "node_modules"), filepath.Join(modulesDir, "node_modules"))
+	if err != nil {
+		return err
+	}
+
+	err = os.Symlink(filepath.Join(modulesDir, "node_modules"), filepath.Join(workingDir, "node_modules"))
+	if err != nil {
+		return err
+	}
+
 	return nil
 }