Fix up replace directives

Courtesy floren. Fixes building u-root when external commands
like edwood are added. edwood has replace directives in go.mod
and they need to be handled correctly.

Signed-off-by: Ronald Minnich <rminnich@gmail.com>

src/pkg/bb/bb.go

@@ -535,6 +535,25 @@ func dealWithDeps(env golang.Environ, bbDir, tmpDir, pkgDir string, mainPkgs []*
 			if err := mod.AddReplace(mpath, "", path.Join("..", "..", mpath), ""); err != nil {
 				return fmt.Errorf("could not add replace rule for %v to go.mod: %v", mpath, err)
 			}
+			// Also copy over every replace directive
+			if module.GoMod != "" {
+				gomodData, err := ioutil.ReadFile(module.GoMod)
+				if err != nil {
+					return err
+				}
+				gomod, err := modfile.Parse(module.GoMod, gomodData, nil)
+				if err != nil {
+					return err
+				}
+				for _, r := range gomod.Replace {
+					if strings.HasPrefix(r.New.Path, "./") || strings.HasPrefix(r.New.Path, "../") || strings.HasPrefix(r.New.Path, "/") {
+						continue
+					}
+					if err := mod.AddReplace(r.Old.Path, r.Old.Version, r.New.Path, r.New.Version); err != nil {
+						return err
+					}
+				}
+			}
 		}
 
 		gomod, err := mod.Format()