fix: use path instead of filepath to join http route paths

[zepatrik]

Windows users were broken since #2292, as routes were registered with backslashes.

See report in https://ory-community.slack.com/archives/C012RJ2MQ1H/p1649677432507569?thread_ts=1649658406.402509&cid=C012RJ2MQ1H

[codecov]

Codecov Report

Merging #2387 (b5b4ba7) into master (9519978) will decrease coverage by 0.08%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #2387      +/-   ##
==========================================
- Coverage   76.60%   76.51%   -0.09%     
==========================================
  Files         315      315              
  Lines       17320    17320              
==========================================
- Hits        13268    13253      -15     
- Misses       3119     3130      +11     
- Partials      933      937       +4     

Impacted Files	Coverage Δ
x/router.go	68.18% <60.00%> (ø)
x/redir.go	100.00% <100.00%> (ø)
courier/smtp.go	60.57% <0.00%> (-10.58%)	⬇️
courier/courier_dispatcher.go	52.94% <0.00%> (-3.93%)	⬇️
session/test/persistence.go	98.61% <0.00%> (-1.39%)	⬇️
persistence/sql/persister_courier.go	88.33% <0.00%> (+3.33%)	⬆️

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