Initialize Steps with a NewSTEPNAME function [#67]

This seems to be be a more natural separation of concerns--the knowledge
of which config fields map to which parts of a Step belong to the Step,
not to the Plan.

internal/run/depupdate.go

@@ -2,6 +2,7 @@ package run
 
 import (
 	"fmt"
+	"github.com/pelotech/drone-helm3/internal/env"
 )
 
 // DepUpdate is an execution step that calls `helm dependency update` when executed.
@@ -10,6 +11,13 @@ type DepUpdate struct {
 	cmd   cmd
 }
 
+// NewDepUpdate creates a DepUpdate using fields from the given Config. No validation is performed at this time.
+func NewDepUpdate(cfg env.Config) *DepUpdate {
+	return &DepUpdate{
+		Chart: cfg.Chart,
+	}
+}
+
 // Execute executes the `helm upgrade` command.
 func (d *DepUpdate) Execute(_ Config) error {
 	return d.cmd.Run()