[training] Use nf-core/modules for the local modules in hello/nf-core

[maxulysse]

For improvment of hello nf-core training:

• https://github.com/nf-core/modules/blob/master/modules/nf-core/cowpy/main.nf
• https://github.com/nf-core/modules/blob/master/modules/nf-core/cat/cat/main.nf