pkg/util/{filepath.go,shortid_generator.go}: Fix golint issues

See,
$ gometalinter --vendor --deadline 10m --disable-all --enable=golint  ./...
filepath.go:12:5⚠️ error var WalkSkipDir should have name of the form ErrFoo (golint)
shortid_generator.go:11:5⚠️ var validUidPattern should be validUIDPattern (golint)
shortid_generator.go:19:6⚠️ func IsValidShortUid should be IsValidShortUID (golint)
shortid_generator.go:24:6⚠️ func GenerateShortUid should be GenerateShortUID (golint)

pkg/middleware/dashboard_redirect_test.go

@@ -20,7 +20,7 @@ func TestMiddlewareDashboardRedirect(t *testing.T) {
 		fakeDash.Id = 1
 		fakeDash.FolderId = 1
 		fakeDash.HasAcl = false
-		fakeDash.Uid = util.GenerateShortUid()
+		fakeDash.Uid = util.GenerateShortUID()
 
 		bus.AddHandler("test", func(query *m.GetDashboardQuery) error {
 			query.Result = fakeDash