[BUG] Optional source path doesn't work if executing xcodegen from outer directory

[Skoti]

Given a project config:

name: OptionalSourcePath

targets:
  OptionalSourcePath:
    type: application
    platform: iOS
    deploymentTarget: "16.0"
    sources:
      - path: "First"
      - path: "Second"
        optional: true

and a directory structure of:

❯ tree dir
dir
├── First
├── Second
└── project.yml

3 directories, 1 file

calling xcodegen from within dir:

cd dir
xcodegen -s project.yml

generates a correct Xcode project.

However, calling xcodegen from outside of dir:

❯ basename $PWD
Downloads
❯ ls dir
First  project.yml  Second
❯ xcodegen -s dir/project.yml

Generates an Xcode project where the "Second" dir is missing from file tree:

❯ xcodegen --version
Version: 2.39.1