Ensure that python dependencies are installed during upgrade (#1390)

## Changes
The installer.Upgrade() processing did not install Python dependencies.
This resulted in errors such as:

```
ModuleNotFoundError: No module named 'databricks.labs.blueprint'
```

Any new dependencies are now installed during the upgrade process.

Resolves: databrickslabs/ucx#1276

## Tests

The TestUpgraderWorksForReleases test now checks to see if the upgrade
process resulted in the dependencies being installed.

---------

Signed-off-by: Jim.Idle <jimi@idle.ws>

cmd/labs/project/installer.go

@@ -136,6 +136,10 @@ func (i *installer) Upgrade(ctx context.Context) error {
 	if err != nil {
 		return fmt.Errorf("installer: %w", err)
 	}
+	err = i.installPythonDependencies(ctx, ".")
+	if err != nil {
+		return fmt.Errorf("python dependencies: %w", err)
+	}
 	return nil
 }
 

cmd/labs/project/installer_test.go

@@ -403,6 +403,12 @@ func TestUpgraderWorksForReleases(t *testing.T) {
 	newHome := copyTestdata(t, "testdata/installed-in-home")
 	ctx = env.WithUserHomeDir(ctx, newHome)
 
+	// Install stubs for the python calls we need to ensure were run in the
+	// upgrade process.
+	ctx, stub := process.WithStub(ctx)
+	stub.WithStderrFor(`python[\S]+ -m pip install .`, "[mock pip install]")
+	stub.WithStdoutFor(`python[\S]+ install.py`, "setting up important infrastructure")
+
 	py, _ := python.DetectExecutable(ctx)
 	py, _ = filepath.Abs(py)
 	ctx = env.Set(ctx, "PYTHON_BIN", py)
@@ -420,4 +426,17 @@ func TestUpgraderWorksForReleases(t *testing.T) {
 
 	r := internal.NewCobraTestRunnerWithContext(t, ctx, "labs", "upgrade", "blueprint")
 	r.RunAndExpectOutput("setting up important infrastructure")
+
+	// Check if the stub was called with the 'python -m pip install' command
+	pi := false
+	for _, call := range stub.Commands() {
+		if strings.HasSuffix(call, "-m pip install .") {
+			pi = true
+			break
+		}
+	}
+	if !pi {
+		t.Logf(`Expected stub command 'python[\S]+ -m pip install .' not found`)
+		t.FailNow()
+	}
 }