Support plugin-level include/exclude configuration per folder (e.g., ignore test/ for certain plugins)

[AndrewDongminYoo]

Describe the problem

At the moment, custom_lint runs all active plugins globally under the root context.
This means there’s no way to disable or scope a plugin (like riverpod_lint) to only part of the codebase, e.g. lib/** but not test/**.

While it’s possible to add a test/analysis_options.yaml, most IDEs (VS Code, IntelliJ) still merge the root configuration and show diagnostics for plugins that should be disabled.
Excluding test/** at the analyzer level disables all lints and refactorings, which is not practical.

This limitation has been discussed in related issues such as:

• workspace's analysis_options.yaml is ignored #364 — workspace’s analysis_options.yaml ignored
• Feature: File-Level Analysis Filtering for CI Optimization #365 — file-level analysis filtering

These clearly show that finer-grained scoping is a recurring need.

Proposed solution

Allow each plugin to define optional include / exclude globs in analysis_options.yaml.
Example:

custom_lint:
  plugins:
    - riverpod_lint:
        include:
          - lib/**
        exclude:
          - test/**
    - company_internal_lints:
        exclude:
          - example/**

This would not affect how rules are defined — it simply changes where each plugin runs.
It could be implemented by checking file path filters when building the analysis context roots or when dispatching diagnostics per plugin.

Why this matters

• Reduces noise from lints that don’t make sense in tests or generated files.
• Avoids breaking refactoring/navigation by globally excluding test folders.
• Keeps the project single-root (no need for separate analyzer contexts).
• Makes custom_lint more flexible for large monorepos and multi-plugin environments.

Alternatives considered

• Using separate analysis_options.yaml per folder (not reliable across IDEs).
• Rule-level ignore_test_files options (moves complexity to each package).
• Excluding test folders globally (breaks analysis and tooling features).

Additional context

This proposal originated from a discussion in riverpod_lint where the maintainer confirmed that folder-based configuration is not currently possible.
Having a plugin-level include/exclude feature in custom_lint would solve this generically for all lint packages.

[rrousselGit]

Plugins are opt-in. If you do not install a plugin for a given package, said package shouldn't be receiving diagnostics from this plugin.

Did you somehow install a plugin in a package that doesn't use it?

[rrousselGit]

To be fair, there are most definitely a couple of bugs in custom_lint here. My guess is you spotted a bug. There shouldn't be a need for anything specific.

The main issue is that an official replacement is being built by Google and I don't really have time to maintain custom_lint.