Improve handling of classpaths in Gradle plugin

[3flex]

This change achieves a few things:

• Allows adding multiple dependencies to the detektPlugins configuration which isn't currently possible (refer Allow custom report format to be defined by Gradle Plugin #1596 for details)
• Improves up-to-date checks by giving Gradle more information about the classpaths used by the tasks
• As a result, fixes Project dependency to override detekt-cli in composite build does not run assemble automatically #1208

This change should not impact anyone's existing build configuration.

cc: @marschwar

[marschwar]

The code looks nice and clean to me.

Is there a use case for users of the detekt plugin to set the detektClasspath or is that mainly to be able to build the plugin against the latest cli?
I am just starting to get a feel for the dependency management with the gradle plugin, so please excuse my ignorance.

[3flex]

Is there a use case for users of the detekt plugin to set the detektClasspath

Usually, no, I expect users would just use the defaults and possibly change the version of the CLI dependency using toolVersion. It's more important for the task so it does proper up-to-date checking.

[codecov-io]

Codecov Report

Merging #1609 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1609   +/-   ##
=========================================
  Coverage     79.62%   79.62%           
  Complexity     1963     1963           
=========================================
  Files           335      335           
  Lines          5527     5527           
  Branches       1014     1014           
=========================================
  Hits           4401     4401           
  Misses          593      593           
  Partials        533      533

---

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[codecov-io]

Codecov Report

Merging #1609 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #1609   +/-   ##
========================================
  Coverage      76.2%   76.2%           
  Complexity     1841    1841           
========================================
  Files           318     318           
  Lines          5456    5456           
  Branches       1007    1007           
========================================
  Hits           4158    4158           
  Misses          754     754           
  Partials        544     544

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[arturbosch]

This looks finish to me, this PR just needs CI restart? Correct me if I'm wrong.

[3flex]

Now has merge conflicts, I can rebase so will rerun CI.

[arturbosch]

Yes please, sorry for the trouble.