refactor(internal/librarian): introduce SourceConfig for structured source root handling (#4255)

Introduce the `SourceConfig` type in the `internal/sidekick/source`
package to replace raw map-based source root tracking. This commit adds:
    - `SourceConfig` struct for structured source root configuration.
- `Resolve` method to handle absolute path resolution across active
roots.
    - Unit tests for new methods.

Author: jameslynnwu

internal/sidekick/config/source_config.go

@@ -0,0 +1,76 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// Sources contains the directory paths for source repositories used by
+// sidekick.
+type Sources struct {
+	Conformance string
+	Discovery   string
+	Googleapis  string
+	ProtobufSrc string
+	Showcase    string
+}
+
+// SourceConfig holds the configuration for source roots and path resolution.
+type SourceConfig struct {
+	Sources     Sources
+	ActiveRoots []string
+	IncludeList []string
+	ExcludeList []string
+}
+
+// Root returns the directory path for the given root name.
+func (c SourceConfig) Root(name string) (string, error) {
+	switch name {
+	case "googleapis":
+		return c.Sources.Googleapis, nil
+	case "discovery":
+		return c.Sources.Discovery, nil
+	case "showcase":
+		return c.Sources.Showcase, nil
+	case "protobuf", "protobuf-src":
+		return c.Sources.ProtobufSrc, nil
+	case "conformance":
+		return c.Sources.Conformance, nil
+	default:
+		return "", fmt.Errorf("unknown source name: %s", name)
+	}
+}
+
+// Resolve returns an absolute path for the given relative path if it is found
+// within the active source roots. Otherwise, it returns the original path.
+func (c SourceConfig) Resolve(relPath string) (string, error) {
+	for _, root := range c.ActiveRoots {
+		rootPath, err := c.Root(root)
+		if err != nil {
+			return "", err
+		}
+		if rootPath == "" {
+			continue
+		}
+		fullName := filepath.Join(rootPath, relPath)
+		if stat, err := os.Stat(fullName); err == nil && !stat.IsDir() {
+			return fullName, nil
+		}
+	}
+	return relPath, nil
+}

internal/sidekick/config/source_config_test.go

@@ -0,0 +1,124 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestRoot(t *testing.T) {
+	cfg := SourceConfig{
+		Sources: Sources{
+			Googleapis: "googleapis-path",
+			Discovery:  "discovery-path",
+		},
+	}
+	for _, test := range []struct {
+		name    string
+		root    string
+		want    string
+		wantErr bool
+	}{
+		{
+			name: "googleapis",
+			root: "googleapis",
+			want: "googleapis-path",
+		},
+		{
+			name: "discovery",
+			root: "discovery",
+			want: "discovery-path",
+		},
+		{
+			name:    "unknown",
+			root:    "unknown",
+			wantErr: true,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := cfg.Root(test.root)
+			if (err != nil) != test.wantErr {
+				t.Fatalf("Root(%q) error = %v, wantErr %v", test.root, err, test.wantErr)
+			}
+			if diff := cmp.Diff(test.want, got); diff != "" {
+				t.Errorf("Root(%q) mismatch (-want +got):\n%s", test.root, diff)
+			}
+		})
+	}
+}
+
+func TestResolve(t *testing.T) {
+	tempDir := t.TempDir()
+	googleapis := filepath.Join(tempDir, "googleapis")
+	if err := os.Mkdir(googleapis, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	specPath := "google/cloud/secretmanager/v1/secretmanager.yaml"
+	fullPath := filepath.Join(googleapis, specPath)
+	if err := os.MkdirAll(filepath.Dir(fullPath), 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(fullPath, []byte("test"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg := SourceConfig{
+		Sources: Sources{
+			Googleapis: googleapis,
+		},
+		ActiveRoots: []string{"googleapis"},
+	}
+
+	for _, test := range []struct {
+		name    string
+		relPath string
+		want    string
+		wantErr bool
+	}{
+		{
+			name:    "found",
+			relPath: specPath,
+			want:    fullPath,
+		},
+		{
+			name:    "not found",
+			relPath: "not/found",
+			want:    "not/found",
+		},
+		{
+			name:    "unknown root",
+			relPath: specPath,
+			wantErr: true,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			if test.name == "unknown root" {
+				cfg.ActiveRoots = []string{"unknown"}
+			}
+			got, err := cfg.Resolve(test.relPath)
+			if (err != nil) != test.wantErr {
+				t.Fatalf("Resolve(%q) error = %v, wantErr %v", test.relPath, err, test.wantErr)
+			}
+			if diff := cmp.Diff(test.want, got); diff != "" {
+				t.Errorf("Resolve(%q) mismatch (-want +got):\n%s", test.relPath, diff)
+			}
+		})
+	}
+}