fix(plugins): support installing plugins by relative path (helm#3568)

Support installing plugins by relative path

```
helm plugins install .
```

pkg/plugin/installer/installer.go

@@ -21,9 +21,9 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"strings"
 
 	"k8s.io/helm/pkg/helm/helmpath"
-	"strings"
 )
 
 // ErrMissingMetadata indicates that plugin.yaml is missing.

pkg/plugin/installer/local_installer.go

@@ -16,6 +16,7 @@ limitations under the License.
 package installer // import "k8s.io/helm/pkg/plugin/installer"
 
 import (
+	"fmt"
 	"path/filepath"
 
 	"k8s.io/helm/pkg/helm/helmpath"
@@ -28,8 +29,12 @@ type LocalInstaller struct {
 
 // NewLocalInstaller creates a new LocalInstaller.
 func NewLocalInstaller(source string, home helmpath.Home) (*LocalInstaller, error) {
+	src, err := filepath.Abs(source)
+	if err != nil {
+		return nil, fmt.Errorf("unable to get absolute path to plugin: %v", err)
+	}
 	i := &LocalInstaller{
-		base: newBase(source, home),
+		base: newBase(src, home),
 	}
 	return i, nil
 }
@@ -41,11 +46,7 @@ func (i *LocalInstaller) Install() error {
 	if !isPlugin(i.Source) {
 		return ErrMissingMetadata
 	}
-	src, err := filepath.Abs(i.Source)
-	if err != nil {
-		return err
-	}
-	return i.link(src)
+	return i.link(i.Source)
 }
 
 // Update updates a local repository