[bug] If packageDir does not exist, run Install (#235)

* If packageDir does not exist, run Install

* Also consider PYTHONUSERBASE in GetPackageDir

* upm show-package-dir should show the root, not site-packages

internal/backends/python/python.go

@@ -284,6 +284,11 @@ func makePythonPoetryBackend(python string) api.LanguageBackend {
 				return venv
 			}
 
+			// Take PYTHONUSERBASE into consideration, if set
+			if userbase := os.Getenv("PYTHONUSERBASE"); userbase != "" {
+				return userbase
+			}
+
 			// Terminate early if we're running inside a repl.
 			// This will suppress the following poetry commands
 			// from showing up in the Packager pane.
@@ -417,9 +422,14 @@ func makePythonPipBackend(python string) api.LanguageBackend {
 				return venv
 			}
 
+			// Take PYTHONUSERBASE into consideration, if set
+			if userbase := os.Getenv("PYTHONUSERBASE"); userbase != "" {
+				return userbase
+			}
+
 			if outputB, err := util.GetCmdOutputFallible([]string{
 				"python",
-				"-c", "import site; print(site.USER_SITE)",
+				"-c", "import site; print(site.USER_BASE)",
 			}); err == nil {
 				return string(outputB)
 			}

internal/cli/cmds.go

@@ -247,7 +247,11 @@ func maybeInstall(ctx context.Context, b api.LanguageBackend, forceInstall bool)
 		if !util.Exists(b.Specfile) {
 			return
 		}
-		if forceInstall || store.HasSpecfileChanged(b) {
+		var needsPackageDir bool
+		if packageDir := b.GetPackageDir(); packageDir != "" {
+			needsPackageDir = !util.Exists(packageDir)
+		}
+		if forceInstall || store.HasSpecfileChanged(b) || needsPackageDir {
 			b.Install(ctx)
 		}
 	}