add explanation in readme, use go mod edit -require

cmd/builder/README.md

@@ -154,10 +154,10 @@ to only execute the compilation step.
 
 ### Avoiding the use of a new go.mod file
 
-You can optionally skip creating a new `go.mod` file.
-When the `--skip-new-go-module` command-line flag is supplied,
-the build process issues a `go get` command for each component,
-relying on the Go toolchain to update the enclosing Go module
+You can optionally skip creating a new `go.mod` file. This is helpful when 
+using a monorepo setup with a shared go.mod file. When the `--skip-new-go-module` 
+command-line flag is supplied, the build process issues a `go get` command for 
+each component, relying on the Go toolchain to update the enclosing Go module 
 appropriately.
 
 This command will avoid downgrading a dependency in the enclosing

cmd/builder/internal/builder/main.go

@@ -264,17 +264,18 @@ func (c *Config) updateModules() error {
 }
 
 func (c *Config) updateGoModule(modspec string) error {
+	mod, ver, found := strings.Cut(modspec, " ")
+	if !found {
+		return fmt.Errorf("ill-formatted modspec %q: missing space separator", modspec)
+	}
+
 	// Re-parse the go.mod file on each iteration, since it can
 	// change each time.
 	modulePath, dependencyVersions, err := c.readGoModFile()
 	if err != nil {
 		return err
 	}
 
-	mod, ver, found := strings.Cut(modspec, " ")
-	if !found {
-		return fmt.Errorf("ill-formatted modspec %q: missing space separator", modspec)
-	}
 	if mod == modulePath {
 		// this component is part of the same module, nothing to update.
 		return nil
@@ -295,9 +296,9 @@ func (c *Config) updateGoModule(modspec string) error {
 	}
 
 	// upgrading or changing version
-	updatespec := mod + "@" + ver
+	updatespec := "-require=" + mod + "@" + ver
 
-	if _, err := runGoCommand(*c, "get", updatespec); err != nil {
+	if _, err := runGoCommand(*c, "mod", "edit", updatespec); err != nil {
 		return err
 	}
 	return nil