Extract route definitions into a separate file

[asvetlov]

A part of #2766 reorganization
web_urldispatcher.py is very huge.

Route definitions could be extracted into a separate web_routedef.py easy.
No tests are broken.
Anyway these names are part of aiohttp.web namespace, importing them directly from aiohttp.web_urldispatcher is an error.

[codecov-io]

Codecov Report

Merging #2876 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2876      +/-   ##
==========================================
+ Coverage   97.97%   97.97%   +<.01%     
==========================================
  Files          39       40       +1     
  Lines        7464     7469       +5     
  Branches     1310     1310              
==========================================
+ Hits         7313     7318       +5     
  Misses         48       48              
  Partials      103      103

Impacted Files	Coverage Δ
aiohttp/web.py	98.8% <100%> (+0.01%)	⬆️
aiohttp/web_urldispatcher.py	99.1% <100%> (-0.12%)	⬇️
aiohttp/web_routedef.py	100% <100%> (ø)

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[new issue]: https://github.com/aio-libs/aiohttp/issues/new