extractors/bilibili: refactor data extraction

[iawia002]

fix #370

[codecov]

Codecov Report

Merging #371 into master will decrease coverage by 0.73%.
The diff coverage is 64.51%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   71.68%   70.95%   -0.74%     
==========================================
  Files          29       29              
  Lines        2084     2107      +23     
==========================================
+ Hits         1494     1495       +1     
- Misses        428      447      +19     
- Partials      162      165       +3

Impacted Files	Coverage Δ
downloader/downloader.go	35.46% <0%> (+0.17%)	⬆️
extractors/bilibili/bilibili.go	68.94% <66.11%> (-7.98%)	⬇️

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