Modular sitemap spider

[stranac]

Currently, the structure of SitemapSpider makes extraction of additional data from a sitemap unnecessarily complicated.
It would require copy-pasting the 22-line _parse_sitemap method, and then processing the created sitemap in a for loop.

This PR splits _parse_sitemap functionality into 3 additional methods, making this kind of processing simpler.
It also moves the link extraction inside the spider.

I also added tests for some of the previously untested SitemapSpider functionality.

[codecov]

Codecov Report

Merging #3543 into master will increase coverage by 0.04%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #3543      +/-   ##
==========================================
+ Coverage   84.38%   84.43%   +0.04%     
==========================================
  Files         167      167              
  Lines        9376     9385       +9     
  Branches     1392     1393       +1     
==========================================
+ Hits         7912     7924      +12     
+ Misses       1206     1204       -2     
+ Partials      258      257       -1

Impacted Files	Coverage Δ
scrapy/spiders/sitemap.py	82.6% <84.61%> (+7.6%)	⬆️