feat(install): skip dependencies

Allows `installing` a package without pulling dependencies of that
package. This comes in handy when having multiple directories on
`JPATH`, which overlay each other.

Because dependencies may be already present at lesser ranked import
paths, it's not required to pull them, when only needing to bump a
single one.

pkg/packages.go

@@ -243,6 +243,11 @@ func ensure(direct map[string]deps.Dependency, vendorDir string, locks map[strin
 	}
 
 	for _, d := range deps {
+		if d.Single {
+			// skip dependencies that explicitely don't want nested ones installed
+			continue
+		}
+
 		f, err := jsonnetfile.Load(filepath.Join(vendorDir, d.Name(), jsonnetfile.File))
 		if err != nil {
 			if os.IsNotExist(err) {