fix(webpack_bundle): support multiple webpack configs without merging

[acozzette]

Currently webpack_bundle and webpack_devserver provide a generated base config, and if the user supplies an additional config, they merge the two by adding the --merge flag. This is not ideal, because the user may wish to have multiple configs.

This change fixes the problem by passing only the generated config to the webpack CLI. This config now loads the user's config (if any), and does any necessary merging.

Fixes #219.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Support multiple webpack configs without merging.

Test plan

• Covered by existing test cases
• New test cases added

[aspect-workflows]

Aspect Workflows Tasks

📅 Wed Oct 14 17:27:04 UTC 2026

Task Results

• ✅ buildifier · complete (clean) · 28s · GitHub Actions · Check
• ✅ gazelle · complete (clean) · 26.7s · GitHub Actions · Check
• ✅ test-e2e-loaders-bazel-7 [test] · complete (1/1 passed · 1 cached) · 3m 5s · Aspect · GitHub Actions · Check
• ✅ test-e2e-loaders-bazel-8 [test] · complete (1/1 passed · 1 cached) · 1m 17s · Aspect · GitHub Actions · Check
• ✅ test-e2e-loaders-bazel-9 [test] · complete (1/1 passed · 1 cached) · 3m 41s · Aspect · GitHub Actions · Check
• ✅ test-e2e-loaders-bazel-9-no-execroot [test] · complete (1/1 passed) · 3m 46s · Aspect · GitHub Actions · Check
• ✅ test-e2e-loaders_jslib-bazel-7 [test] · complete (1/1 passed · 1 cached) · 2m 18s · Aspect · GitHub Actions · Check
• ✅ test-e2e-loaders_jslib-bazel-8 [test] · complete (1/1 passed · 1 cached) · 53.6s · Aspect · GitHub Actions · Check
• ✅ test-e2e-loaders_jslib-bazel-9 [test] · complete (1/1 passed · 1 cached) · 2m 10s · Aspect · GitHub Actions · Check
• ✅ test-e2e-loaders_jslib-bazel-9-no-execroot [test] · complete (1/1 passed · 1 cached) · 2m 56s · Aspect · GitHub Actions · Check
• ✅ test-e2e-path_mapping-bazel-7 [test] · complete (1/1 passed · 1 cached) · 2m 18s · Aspect · GitHub Actions · Check
• ✅ test-e2e-path_mapping-bazel-8 [test] · complete (1/1 passed · 1 cached) · 45.9s · Aspect · GitHub Actions · Check
• ✅ test-e2e-path_mapping-bazel-9 [test] · complete (1/1 passed) · 2m 47s · Aspect · GitHub Actions · Check
• ✅ test-e2e-path_mapping-bazel-9-no-execroot [test] · complete (1/1 passed · 1 cached) · 2m 54s · Aspect · GitHub Actions · Check
• ✅ test-e2e-repo_mapping-bazel-7 [test] · complete (1/1 passed · 1 cached) · 34.8s · Aspect · GitHub Actions · Check
• ✅ test-e2e-repo_mapping-bazel-8 [test] · complete (1/1 passed · 1 cached) · 34.3s · Aspect · GitHub Actions · Check
• ✅ test-e2e-repo_mapping-bazel-9 [test] · complete (1/1 passed) · 50.1s · Aspect · GitHub Actions · Check
• ✅ test-e2e-repo_mapping-bazel-9-no-execroot [test] · complete (1/1 passed · 1 cached) · 56.4s · Aspect · GitHub Actions · Check
• ✅ test-e2e-smoke-bazel-7 [test] · complete (3/3 passed · 3 cached) · 2m 47s · Aspect · GitHub Actions · Check
• ✅ test-e2e-smoke-bazel-8 [test] · complete (3/3 passed · 3 cached) · 3m 8s · Aspect · GitHub Actions · Check
• ✅ test-e2e-smoke-bazel-9 [test] · complete (3/3 passed · 1 cached) · 6m 18s · Aspect · GitHub Actions · Check
• ✅ test-e2e-smoke-bazel-9-no-execroot [test] · complete (3/3 passed · 2 cached) · 5m 15s · Aspect · GitHub Actions · Check
• ✅ test-e2e-worker-bazel-7 [test] · complete (2/2 passed · 2 cached) · 2m 32s · Aspect · GitHub Actions · Check
• ✅ test-e2e-worker-bazel-8 [test] · complete (2/2 passed · 2 cached) · 2m 59s · Aspect · GitHub Actions · Check
• ✅ test-e2e-worker-bazel-9 [test] · complete (2/2 passed · 2 cached) · 3m 52s · Aspect · GitHub Actions · Check
• ✅ test-e2e-worker-bazel-9-no-execroot [test] · complete (2/2 passed) · 2m 33s · Aspect · GitHub Actions · Check
• ✅ test-root-bazel-7 [test] · complete (20/20 passed · 12 cached) · 3m 36s · Aspect · GitHub Actions · Check
• ✅ test-root-bazel-8 [test] · complete (18/18 passed · 11 cached) · 1m 56s · Aspect · GitHub Actions · Check
• ✅ test-root-bazel-9 [test] · complete (18/18 passed · 16 cached) · 3m 48s · Aspect · GitHub Actions · Check
• ✅ test-root-bazel-9-no-execroot [test] · complete (18/18 passed · 2 cached) · 3m 25s · Aspect · GitHub Actions · Check

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