Refactoring TopicReader, exposing background link topics from TREC 2020 News Track

[lintool]

Better way of building TOPIC_FILE_TO_TYPE mapping so we don't have to enter same information twice.

[codecov]

Codecov Report

Merging #1502 (f6a2f16) into master (346cb9b) will decrease coverage by 0.21%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1502      +/-   ##
============================================
- Coverage     57.79%   57.57%   -0.22%     
- Complexity      977      978       +1     
============================================
  Files           166      166              
  Lines          9053     9007      -46     
  Branches       1226     1227       +1     
============================================
- Hits           5232     5186      -46     
  Misses         3431     3431              
  Partials        390      390              

Impacted Files	Coverage Δ	Complexity Δ
...va/io/anserini/search/topicreader/TopicReader.java	63.46% <100.00%> (-17.35%)	12.00 <2.00> (+1.00)	⬇️
...in/java/io/anserini/search/topicreader/Topics.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)

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