Moved BuildInfo into package mill.contrib.buildinfo

This fixes #548

See also #547

contrib/buildinfo/src/BuildInfo.scala

@@ -1,4 +1,4 @@
-package mill.contrib
+package mill.contrib.buildinfo
 
 import mill.T
 import mill.define.Target

contrib/buildinfo/src/deprecated.scala

@@ -0,0 +1,4 @@
+package mill.contrib
+
+@deprecated("Moved to package mill.contrib.buildinfo", "0.3.7")
+trait BuildInfo extends mill.contrib.buildinfo.BuildInfo

contrib/buildinfo/test/src/BuildInfoTests.scala

@@ -1,4 +1,4 @@
-package mill.contrib
+package mill.contrib.buildinfo
 
 import java.util.jar.JarFile
 import mill._

docs/pages/9 - Contrib Modules.md

@@ -5,10 +5,15 @@
 Generate scala code from your buildfile.
 This plugin generates a single object containing information from your build.
 
-To declare a module that uses BuildInfo you must extend the `mill.contrib.BuildInfo` trait when defining your module.
+To declare a module that uses BuildInfo you must extend the `mill.contrib.buildinfo.BuildInfo` trait when defining your module.
 
 Quickstart:
 ```scala
+// build.sc
+// You have to replace VERSION
+import $ivy.`com.lihaoyi::mill-contrib-buildinfo:VERSION`
+import mill.contrib.buildinfo.BuildInfo
+
 object project extends BuildInfo {
   val name = "poject-name"
   def  buildInfoMembers: T[Map[String, String]] = T {