Add-ons

[wRAR]

This is #1272, updated to the current code and with builtin-addons removed. There should be some unanswered questions in that PR. I've addressed some of the simplest feedback and made the CI pass, though there are still documentation references to file paths and there may be other things that need to be fixed.

I haven't thought much about the implementation itself yet, but I already have a question "how can a user override a priority of a middleware set by an addon or replace that middleware with their own subclass of it".

Closes: #1272, fixes #5992

[codecov]

Codecov Report

Merging #5950 (d75ea3d) into master (5105742) will increase coverage by 0.04%.
Report is 34 commits behind head on master.
The diff coverage is 93.29%.

❗ Current head d75ea3d differs from pull request most recent head e58b807. Consider uploading reports for the commit e58b807 to get more accurate results

@@            Coverage Diff             @@
##           master    #5950      +/-   ##
==========================================
+ Coverage   89.19%   89.23%   +0.04%     
==========================================
  Files         162      163       +1     
  Lines       11287    11369      +82     
  Branches     1833     1844      +11     
==========================================
+ Hits        10067    10145      +78     
- Misses        928      930       +2     
- Partials      292      294       +2     

Files Changed	Coverage Δ
scrapy/middleware.py	98.18% <ø> (+5.32%)	⬆️
scrapy/utils/misc.py	96.55% <ø> (ø)
scrapy/cmdline.py	68.70% <75.00%> (+0.73%)	⬆️
scrapy/addons.py	84.00% <84.00%> (ø)
scrapy/settings/__init__.py	95.40% <91.04%> (-2.94%)	⬇️
scrapy/crawler.py	87.56% <100.00%> (+0.18%)	⬆️
scrapy/exporters.py	100.00% <100.00%> (ø)
scrapy/extensions/closespider.py	96.82% <100.00%> (+1.70%)	⬆️
scrapy/extensions/feedexport.py	95.65% <100.00%> (-0.17%)	⬇️
scrapy/settings/default_settings.py	98.78% <100.00%> (+<0.01%)	⬆️
... and 2 more

... and 2 files with indirect coverage changes

[wRAR]

There is a big inconsistency problem nere: "addon" vs "add-on".

[Gallaecio]

Yes, it would be great if we could normalize it as ”add-on” outside code. No strong opinion on the code side (AddOn vs Addon).

[wRAR]

Thanks!

[jdemaeyer]

🤯