Support for multiple configuration files in a directory

[juhoinkinen]

Adds support for a project configuration directory. Closes #584.

Checks for duplicated project IDs across all configuration files in the configuration directory, and raises ConfigurationException if any are found.

For now I chose projects.d as the name for the default configuration directory, because it is nicely in line with the projects.cfg and projects.toml names for the default configuration files. Downside is that this name can be confusing: one could use "projects directory" to refer to the (projects) configuration directory (projects.d) or the projects data directory (data/projects). Or even to the parent directory containing projects.cfg and data/ (in Docker instances I have been using /annif-projects directory for this).

I open PR as draft now; this probably needs some more tests etc.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
6 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #585 (1883de8) into master (aff0411) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   99.47%   99.48%           
=======================================
  Files          84       84           
  Lines        5568     5615   +47     
=======================================
+ Hits         5539     5586   +47     
  Misses         29       29           

Impacted Files	Coverage Δ
annif/cli.py	99.63% <100.00%> (ø)
annif/config.py	100.00% <100.00%> (ø)
annif/default_config.py	100.00% <100.00%> (ø)
annif/registry.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_config.py	100.00% <100.00%> (ø)
tests/test_project.py	100.00% <100.00%> (ø)

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[juhoinkinen]

One reason to use the projects.d name instead of conf, config etc. is that the latter names make to think about some general configuration not necessarily about projects (like log file location, parallelization settings... although those are not existing in Annif now, maybe some day some like those could be).

I don't see anything to add to this now, @osma can you take a look?

[osma]

Code looks good!
I tested this briefly and couldn't make it not work ;)
Great work!

[osma]

I edited the opening post slightly ("closing #584" -> "closes #584") so that GitHub recognizes the relationship and will automatically close the issue when this PR is merged.