Upgrade Zinc and fix over-compilation (#390)

* Remove dead code

* Upgrade zinc to 1.1.7, avoid hardcoding the version

* Fix over-compilation with Zinc

`upstreamCompileOutput` needs to contain the CompileOutput of all
recursive dependencies, otherwise Zinc gets confused when a classfile
from one of these recursive dependency is used, resulting in
over-compilation.

No tests because I don't know how to write one, but this can be observed
manually:

1. mill scalajslib.compile
2. Add an empty line to ScalaModule.scala
3. mill scalajslib.compile

Before this commit, the last compilation ended up compiling one file in
scalalib (as expected), then every file in scalajslib. After this
commit, nothing in scalajslib is recompiled, as expected.

1 files changed, 6 insertions, 0 deletions

diff --git a/scalalib/src/mill/scalalib/Versions.scala b/scalalib/src/mill/scalalib/Versions.scala
new file mode 100644
index 00000000..6c1c385f
--- /dev/null
+++ b/scalalib/src/mill/scalalib/Versions.scala
@@ -0,0 +1,6 @@
+package mill.scalalib
+
+object Versions {
+  // Keep synchronized with zinc dependency in scalalib.worker in build.sc
+  val zinc = "1.1.7"
+}