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

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

Start by reproducing the behavior with xcodegen -s dir/project.yml, then compare it with running xcodegen from inside dir using the provided project.yml. Trace how the optional Second source path is resolved relative to the specification and working directory. Done means both invocations include Second in the generated Xcode project.