feat: Use multiple workflow files for sub packages

[renancaraujo]

As described on https://github.com/VeryGoodOpenSource/very_good_workflows/issues/31#issuecomment-1127964101, support the following structure of multiple packages in a repository:

<repository root (flutter app)>
| - .github/
    | - workflows/
        | - root_package.yaml
        | - package_check_workflow.yaml
        | - sub_packages.yaml
| - packages/
    | - sub_package_1
        | - .github/
            | - workflows/
                | - sub_package_1.yaml
    | - sub_package_2
        | - .github/
            | - workflows/
                | - sub_package_2.yaml
    | - sub_package_3
        | - .github/
            | - workflows/
                | - sub_package_3.yaml
    | - sub_package_4
        | - .github/
            | - workflows/
                | - sub_package_4.yaml
    | - sub_package_5
        | - .github/
            | - workflows/
                | - sub_package_5.yaml
 ...