Reorganize repository check modules

[ilveroluca]

This PR proposes a solution for issue #246 by reorganizing automated repository check implementations into new modules. The idea is to split the checks into the following categories:

• general.lm: things specific to LifeMonitor
• general.metadata: metadata-related checks (e.g., RO-crate properties)
• general.repo_layout: general repository layout issues, such as missing metadata files or workflow files
• <one category per supported workflow type -- e.g., galaxy, snakemake)>

The lifemonitor/api/models/issues directory therefore takes the form:

./galaxy
./snakemake
./general
./general/metadata.py
./general/repo_layout.py
./general/experimental.py
./general/lm.py
./__init__.py

The PR changes the generation of check identifiers to use the module+class name rather than converting the class name to snake case (the automated conversion did not work very well).

The PR also proposes fixing for some small type annotation issues and a change to the check finding function (the original presented problems when relative imports were used).

[kikkomep]

LGTM!