fix jacoco and enable codecov.io action

[MysterAitch]

fix jacoco reporting (unable to produce the report without having the sources available directly within that submodule), and enable code coverage github action / pr comments

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c0b30c6). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2906   +/-   ##
=========================================
  Coverage          ?   52.19%           
=========================================
  Files             ?      438           
  Lines             ?    23436           
  Branches          ?     3535           
=========================================
  Hits              ?    12232           
  Misses            ?    10291           
  Partials          ?      913           

Flag	Coverage Δ
AlsoSlowTests	52.19% <0.00%> (?)
jdk-10	52.19% <0.00%> (?)
jdk-11	52.18% <0.00%> (?)
jdk-12	52.18% <0.00%> (?)
jdk-13	52.18% <0.00%> (?)
jdk-14	52.18% <0.00%> (?)
jdk-15	52.19% <0.00%> (?)
jdk-8	52.17% <0.00%> (?)
jdk-9	52.19% <0.00%> (?)
macos-latest	52.19% <0.00%> (?)
ubuntu-latest	52.18% <0.00%> (?)
windows-latest	52.18% <0.00%> (?)

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