Move the internal routing code to wvlet.airframe.http.router package

[xerial]

This PR is just for moving internal code to router package. Router object, which is used for defining HTTP endpoints will remain in http package, so it should have no user-facing change.

[codecov]

Codecov Report

Merging #824 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #824   +/-   ##
=======================================
  Coverage   80.95%   80.95%           
=======================================
  Files         224      224           
  Lines        8904     8904           
  Branches      620      614    -6     
=======================================
  Hits         7208     7208           
  Misses       1696     1696

Impacted Files	Coverage Δ
...la/wvlet/airframe/http/finagle/FinagleClient.scala	89.53% <ø> (ø)	⬆️
...la/wvlet/airframe/http/finagle/FinagleFilter.scala	100% <ø> (ø)	⬆️
...n/scala/wvlet/airframe/http/router/Automaton.scala	78.57% <ø> (ø)
...wvlet/airframe/http/router/HttpResponseCodec.scala	54.54% <ø> (ø)
...t/airframe/http/router/HttpRequestDispatcher.scala	95.23% <ø> (ø)
.../main/scala/wvlet/airframe/http/router/Route.scala	68.75% <ø> (ø)
...la/wvlet/airframe/http/finagle/FinagleServer.scala	67.56% <ø> (ø)	⬆️
...airframe/http/finagle/FinagleResponseHandler.scala	82.25% <ø> (ø)	⬆️
...tp/src/main/scala/wvlet/airframe/http/Router.scala	65% <ø> (ø)	⬆️
...wvlet/airframe/http/router/HttpRequestMapper.scala	96.66% <ø> (ø)
... and 2 more

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