Support batch mode for better performance on large codebases

[kszafran]

See https://github.com/uber-go/mock/issues/245 for the motivation behind this PR.

I split this PR into a series of self-contained commits, so that it's easier to review the changes commit-by-commit.

Summary by CodeRabbit

• Documentation

  • Added a "Batch mode" section explaining how to use a YAML configuration file to generate mocks for multiple packages at once.

• New Features

  • Introduced batch processing for mock generation, enabling simultaneous handling of multiple packages with enhanced configuration options and streamlined command-line flag management.

[coderabbitai[bot]]

Walkthrough

This update introduces a new “Batch mode” for the mockgen tool. The documentation now explains how to configure batch processing using a YAML file with both global and package-specific flags. The code base has been refactored to support this feature by adding new constants, helper functions, and data structures for configuration handling. In addition, method signatures in the generator and tests have been updated to incorporate the new batch logic. The package parsing logic has also been modified to handle multiple packages concurrently.

Changes

File(s)	Change Summary
README.md	Added a new "Batch mode" section that documents how to generate mocks for multiple packages simultaneously using a YAML configuration file.
go.mod	Updated dependency management for gopkg.in/yaml.v3: added as a direct dependency and removed it from the indirect dependencies list.
mockgen/mockgen.go, mockgen/mockgen_test.go	Introduced new constants and a helper function prepareTargets to parse the YAML file. Added new types (e.g., Batch, Target, Flags, genTarget) for batch processing. Updated method signatures including Generate and GenerateMockInterface (adding an extra boolean flag) to streamline processing of batch configuration. Test invocation was updated accordingly.
mockgen/package_mode.go	Refactored package parsing to support multiple packages by removing the pkgName field, adding the parsePackages method, and modifying methods for alias replacement and package loading to accept multiple packages.

Sequence Diagram(s)

sequenceDiagram
    participant U as User
    participant M as mockgen Main
    participant PT as prepareTargets
    participant GT as generateTarget
    participant G as generator

    U->>M: Run mockgen with YAML config
    M->>PT: Process YAML to extract batch targets
    PT-->>M: Return list of genTarget objects
    loop For each target
        M->>GT: Call generateTarget(target)
        GT->>G: Invoke Generate(target) / GenerateMockInterface(target)
        G-->>GT: Return generated mock
    end

Poem

Oh, I hopped to spread the news today,
Batch mode is here in a brilliant array!
YAML files dance to set mocks free,
Flags and targets unite in harmony.
I, a happy rabbit, cheer with delight—
CodeRabbit leaps, making mock generation bright!

✨ Finishing Touches

• [ ] 📝 Generate Docstrings

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[CLAassistant]

All committers have signed the CLA.

[ArujReis]

@sywhang, @JacobOaks, @tchung1118, can you take a look at this? This could be a really big improvement.

[kszafran]

Please also take a look at my comment here: https://github.com/uber-go/mock/issues/245#issuecomment-2832577637 for an alternative/supplementary approach.