Rename functions

[peterdesmet]

Fix #263

Our functions all start with a verb, which makes sense, because they perform an action (read, create, remove).

That is less the case for accessor functions (which have a generic "get"). That is why it's more intuitive to just call them after the thing, e.g. schema() rather than get_schema(). We already did this with resources(), which is better named resource_names() as that is what it returns. Future accessor functions could include version().

This PR therefore renames:

1. get_schema() → schema(): a public accessor function => deprecation
2. resources() → resource_names(): a public read accessor function => deprecation
3. get_resource() → resource(): a private accessor function => no deprecation

The filename for print.R is also renamed to print.datapackage.R in preparation for print functions for schema() and resource().

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (34a2d01) to head (bb35a0f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #282   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        23    +1     
  Lines          638       650   +12     
=========================================
+ Hits           638       650   +12     

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[peterdesmet]

Right, I forgot tests for my deprecated functions. Will add that.