Make the gradle plugin a sub project again

[arturbosch]

With the current setup adding a new feature like in #2492 is cumbersome.
Until a new cli version is published this feature cannot be merged or it will fail the build.
Our current approach is good for having the newest gradle plugin changes in a gradle detekt run. However it slows down build time and development time.
Fast feedback can still be provided with the new pluginManagement closure:

pluginManagement {
    repositories {
        maven { setUrl("https://plugins.gradle.org/m2/") }
        mavenLocal() // used to publish and test local gradle plugin changes
    }
...
}

All it needs is to run publishToMavenLocal.

I propose to change it back to a sub project. To verify newest changes we could move to following commands in our CI:

gradle build -x detekt
gradle publishToMavenLocal
gradle detekt

This PR leads to:

• faster build file evaluate, no need to wait for the composite build file evaluation
• cleaner build files, less duplications
• more up-to-date tasks
• faster builds, clean build is ~30 seconds faster

[BraisGabin]

Would this help with #2484?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@f162996). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2493   +/-   ##
=========================================
  Coverage          ?   80.12%           
  Complexity        ?     2253           
=========================================
  Files             ?      378           
  Lines             ?     6648           
  Branches          ?     1176           
=========================================
  Hits              ?     5327           
  Misses            ?      741           
  Partials          ?      580

Impacted Files	Coverage Δ	Complexity Δ
.../main/kotlin/io/gitlab/arturbosch/detekt/Detekt.kt	34.21% <ø> (ø)	19 <0> (?)

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[arturbosch]

Would this help with #2484?

Yes, this should not happen as we use a stable detekt version for self analysis.

[schalkms]

@arturbosch did the test coverage noticeably go down after this commit or after the following one?

Setup for easy local gradle plugin changes

[arturbosch]

@schalkms seems it was this PR -> https://codecov.io/gh/arturbosch/detekt/pull/2493/changes

The gradle plugin is now a sub project and provides coverage data. So it dropped by 3.14% :(

[schalkms]

@arturbosch I also noticed this when I had a free minute. The only thing we can do is to provide separate unit tests for this sub project, I guess. Before, when it wasn't a sub-project, the analyzer went through the paths.