allow git submodule with mustache templates to be used as package data

[Remi-Gau]

No description provided.

[Remi-Gau]

Typical error in CI:

 template_name = 'pet.mustache'
data = {'AcquisitionMode': 'list mode', 'AttenuationCorrection': '[137Cs]transmission scan-based', 'BodyPart': 'Brain', 'FrameDuration': [10, 20, 30, 40, 50, 60, ...], ...}

    def render(template_name=None, data=None):
    
        template_file = Path(__file__).parent.joinpath("templates", "templates", template_name)
    
>       with open(template_file) as template:
E       FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/work/pybids-reports/pybids-reports/bids/ext/reports/templates/templates/pet.mustache'