Add support for Scoverage (#571)

* Add initial work for ScoverageModule

* style: Move package scoverage from lib to contrib

Suggested by @lefou
> I think, it would be better to add under `mill.contrib.scoverage`.

Pull request: #571

* Initial changes to non-hardcoded scoverage version

* Using task context api to specify dataDir

Now measurement data will be written to:
PROJECT_ROOT/out/MODULE/scoverage/data/

and the html report will be written to:
PROJECT_ROOT/out/MODULE/scoverage/data/htmlReport/

* Remove wild card imports in scoverage

Also remove scoverage dependency from build.sc

* Move htmlReport into worker

Based on what I've seen in
scalalib, scalajslib, scalanativelib, playlib, and twirllib modules.

Still need to add tests

* Add basic docs + tests for scoverage

I still am working on testing the actual generation of
reports.

* Use cross-module for scoverage worker

Now we can support multiple versions of scoverage
by adding them to the crossmodule list.

Also now running the local publish script succeeds.

* Add scoverage to ci tests

* Add detailed ScoverageModule documentation

* Test scoverage dataDir

* Remove <pre> tags in scaladoc

* Add scoverage dependency in less hacky way

* Modify scoverage tests to check classpaths

* Put docs in alphabetical order

* Test classpaths for scoverage runtime

* Remove abstract def test: ScoverageTests

* Construct classloader differently

* Revert "Construct classloader differently"

This reverts commit fccf9a94cc38fb9e2be58a9ff90b00b65f339db6.

* Revert "Construct classloader differently"

Also fixes unfound error in html report

This reverts commit fccf9a94cc38fb9e2be58a9ff90b00b65f339db6.

* Fix classpath for scoverage worker

1 files changed, 43 insertions, 1 deletions

diff --git a/docs/pages/9 - Contrib Modules.md b/docs/pages/9 - Contrib Modules.md
index eca61be3..c3d4751a 100644
--- a/docs/pages/9 - Contrib Modules.md
+++ b/docs/pages/9 - Contrib Modules.md
@@ -543,6 +543,49 @@ object example extends ScalaPBModule {
 }
 ```
 
+
+## Scoverage
+
+This module allows you to generate code coverage reports for Scala projects with
+[Scoverage](https://github.com/scoverage) via the
+[scalac-scoverage-plugin](https://github.com/scoverage/scalac-scoverage-plugin).
+
+To declare a module for which you want to generate coverage reports you can
+extends the `mill.contrib.scoverage.ScoverageModule` trait when defining your
+module. Additionally, you must define a submodule that extends the
+`ScoverageTests` trait that belongs to your instance of `ScoverageModule`.
+
+```scala
+// You have to replace VERSION
+import $ivy.`com.lihaoyi::mill-contrib-buildinfo:VERSION`
+import mill.contrib.scoverage.ScoverageModule
+
+object foo extends ScoverageModule  {
+  def scalaVersion = "2.11.8"
+  def scoverageVersion = "1.3.1"
+
+  object test extends ScoverageTests {
+    def ivyDeps = Agg(ivy"org.scalatest::scalatest:3.0.5")
+    def testFrameworks = Seq("org.scalatest.tools.Framework")
+  }
+}
+```
+
+In addition to the normal tasks available to your Scala module, Scoverage
+modules introduce a few new tasks and changes the behavior of an existing one.
+
+```
+mill foo.scoverage.compile      # compiles your module with test instrumentation
+                                # (you don't have to run this manually, running the test task will force its invocation)
+
+mill foo.test                   # tests your project and collects metrics on code coverage
+mill foo.scoverage.htmlReport   # uses the metrics collected by a previous test run to generate a coverage report in html format
+```
+
+The measurement data is available at `out/foo/scoverage/data/`,
+and the html report is saved in `out/foo/scoverage/htmlReport/`.
+
+
 ## TestNG
 
 Provides support for [TestNG](https://testng.org/doc/index.html).
@@ -722,4 +765,3 @@ These imports will always be added to every template.  You don't need to list th
 
 ### Example
 There's an [example project](https://github.com/lihaoyi/cask/tree/master/example/twirl)
-