connectors: refactor filter code into a helper package

I hope I didn't miss any :D

Signed-off-by: Stephan Renatus <srenatus@chef.io>

connector/github/github.go

@@ -20,6 +20,7 @@ import (
 	"golang.org/x/oauth2/github"
 
 	"github.com/dexidp/dex/connector"
+	groups_pkg "github.com/dexidp/dex/pkg/groups"
 	"github.com/dexidp/dex/pkg/log"
 )
 
@@ -375,7 +376,7 @@ func (c *githubConnector) groupsForOrgs(ctx context.Context, client *http.Client
 		// 'teams' list in config.
 		if len(org.Teams) == 0 {
 			inOrgNoTeams = true
-		} else if teams = filterTeams(teams, org.Teams); len(teams) == 0 {
+		} else if teams = groups_pkg.Filter(teams, org.Teams); len(teams) == 0 {
 			c.logger.Infof("github: user %q in org %q but no teams", userName, org.Name)
 		}
 
@@ -466,22 +467,6 @@ func (c *githubConnector) userOrgTeams(ctx context.Context, client *http.Client)
 	return groups, nil
 }
 
-// Filter the users' team memberships by 'teams' from config.
-func filterTeams(userTeams, configTeams []string) (teams []string) {
-	teamFilter := make(map[string]struct{})
-	for _, team := range configTeams {
-		if _, ok := teamFilter[team]; !ok {
-			teamFilter[team] = struct{}{}
-		}
-	}
-	for _, team := range userTeams {
-		if _, ok := teamFilter[team]; ok {
-			teams = append(teams, team)
-		}
-	}
-	return
-}
-
 // get creates a "GET `apiURL`" request with context, sends the request using
 // the client, and decodes the resulting response body into v. A pagination URL
 // is returned if one exists. Any errors encountered when building requests,