Add CuckooPluginModular for per-module mock generation

[SylvainRX]

Summary

Adds a new CuckooPluginModular build tool plugin that generates separate mock files per module dependency, improving modularity for multi-target Swift Packages.

Related issue: #555

Motivation

The existing CuckooPluginSingleFile generates all mocks into a single GeneratedMocks.swift file in derived data. In a Swift Package with multiple targets, each test target compiles independently and may not have visibility into types from unrelated modules, making a single shared mock file problematic.

Changes

New Plugin: CuckooPluginModular

• Located in Generator/Plugin/Modular/
• Inspects the test target's dependencies (excluding Cuckoo) and runs the generator once per dependency module
• Each module gets its own build command with CUCKOO_MODULE_NAME environment variable
• Generates one mock file per module: GeneratedMocks_<ModuleName>.swift
• Also emits a build command keyed by the test target's own name, aggregating all dependency sources — this allows Cuckoofile.toml to have a [modules.<TestTargetName>] entry to control which files are mocked and which imports are added
• A Cuckoofile.toml entry is required for each module; modules without a matching entry produce an empty file

Usage

Package.swift:

.testTarget(
    name: "TargetATests",
    dependencies: ["TargetA", "Cuckoo"],
    plugins: [
        .plugin(name: "CuckooPluginModular", package: "Cuckoo"),
    ]
),
.testTarget(
    name: "AggregationTargetTests",
    dependencies: ["AggregationTarget", "Cuckoo"],
    plugins: [
        .plugin(name: "CuckooPluginModular", package: "Cuckoo"),
    ]
)

Cuckoofile.toml:

# TargetA mocks
[modules.TargetATests]
imports = ["Foundation"]
testableImports = ["TargetA"]
sources = [
    "Sources/TargetA/InternalProtocolA.swift"
]

# AggregationTarget mocks - multiple imports
[modules.AggregationTargetTests]
imports = ["Foundation", "TargetA", "TargetB"]
sources = [
    "Sources/TargetA/ProtocolA.swift",
    "Sources/TargetB/ProtocolB.swift",
]

Breaking Changes

None. All changes are backward compatible with existing plugins and configurations.

[MatyasKriz]

Looks great! I appreciate you taking the time to improve the project like this. Especially the automatic source detection might be nice for small projects.

One more thing, though – please mention this new modular build tool in README, so that users don't miss it. 🙂

[SylvainRX]

@MatyasKriz Thank you for the review and feedbacks! I'll address them as soon as I have some time.

[SylvainRX]

Hello @MatyasKriz !
I pushed changes addressing your comments, and updated the readme as well.
Please let me know if there's thing I can keep improving on.
Thanks!

[MatyasKriz]

Sorry for the delay, looks good! The documentation is a nice touch as well. 👏

[SylvainRX]

Thank you @MatyasKriz !