fix: ensure replaced modules are required in go.mod during build

When using the --replace flag, the build process was skipping go get
for replaced modules but not adding them as requirements, causing
Go compilation errors. This fix ensures replaced modules are properly
required using go mod edit -require with placeholder versions.

Fixes #87

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: andypost

plugins/builder/environment.go

@@ -119,20 +119,44 @@ func (env *buildEnvironment) CreateModFile(ctx context.Context, opts *BuildOptio
 		if err != nil {
 			return err
 		}
+	} else {
+		// If core package is replaced, we still need to require it.
+		// We use go mod edit -require to add the requirement without downloading.
+		// Since the module is replaced, we use a placeholder version.
+		err = env.execGoMod(ctx, "edit", "-require", opts.CorePkg.String()+"@v0.0.0")
+		if err != nil {
+			return err
+		}
 	}
 
 	// Download plugins.
-nextPlugin:
 	for _, p := range opts.Plugins {
-		// Do not get plugins of module subpath.
+		// Check if this plugin is replaced.
+		var pluginReplaced bool
 		for repl := range opts.ModReplace {
 			if strings.HasPrefix(p.Path, repl) {
-				continue nextPlugin
+				pluginReplaced = true
+				break
 			}
 		}
-		err = env.execGoGet(ctx, p.String())
-		if err != nil {
-			return err
+
+		if !pluginReplaced {
+			err = env.execGoGet(ctx, p.String())
+			if err != nil {
+				return err
+			}
+		} else {
+			// If plugin is replaced, we still need to require it.
+			// We use go mod edit -require to add the requirement without downloading.
+			// Since the module is replaced, we use a placeholder version if none specified.
+			pkgStr := p.String()
+			if !strings.Contains(pkgStr, "@") {
+				pkgStr += "@v0.0.0"
+			}
+			err = env.execGoMod(ctx, "edit", "-require", pkgStr)
+			if err != nil {
+				return err
+			}
 		}
 	}
 	// @todo update all but with fixed versions if requested