Fix: include subprojects' ignore and output directories to prevent rebuild loops in watch when using broadcast

[fewwan]

This PR fixes an issue where only the main project’s ignore patterns and output directory were considered when building the final ignore list.
As a result, subprojects’ ignore rules and outputs were not excluded properly when using broadcast.

Why it matters
The ignore list is used by the --watch option to determine which files should trigger rebuilds.
Previously, since subproject build directories were not ignored, any file changes inside them would cause infinite rebuild loops, showing the build triggered repeatedly warning.
This fix prevents that by correctly ignoring all subprojects’ output and ignore paths.

Changes

• Updated the ignore property to:
  • Include each subproject’s ignore list.
  • Include each subproject’s output directory.
• Deduplicates entries using a set while maintaining the same return type.

Example

Project structure

packs/
├── foo/
│   ├── build/
│   └── beet.yml
├── bar/
│   ├── baz/
│   ├── build/
│   └── beet.yml
└── beet.yml

Configs
packs/foo/beet.yml

output: build

packs/bar/beet.yml

output: build
ignore:
  - baz

beet.yml

broadcast: packs/*
extend: [beet.yml]

Before

Changes in any of the following directories would cause an infinite rebuild loop:

packs/foo/build
packs/bar/build
packs/bar/baz

After

All subprojects’ ignore and output directories are properly ignored, and the watch now behaves as expected.

[vberlier]

Thanks for the high-quality PR! I just merged this in the beta branch, which is going to be the next feature version. It's already available on PyPI! https://pypi.org/project/beet/0.113.0b9/