Sort script sources to ensure deterministic generation of child module aliases in the build.mill/package.mill file (#4113)

* The `Seq` returned from `MillBuildRootModule#scriptSources` comes from
the keys of a `Map`, meaning it is arbitrary non-deterministic orders
* Causing the resultant child module aliases (generated in `build.mill`
and `package.mill` files to reference sub-folder `package.mill` files)
to occur in non-deterministic orders
* Resulting in bytecode changes (e.g. varying the bit position of each
reference the lazy val initialization bitmap) that causes lots of stuff
to unnecessarily invalidate

Tested manually, seems to remove the misbehavior reported in
#4112. Managed to reproduce it
in `integration/invalidation/codesig-subfolder` with some tweaks, so I
left the repro in place to guard against regression

This still leaves us open to unnecessary invalidations if someone adds a
new `subfolder/package.mill` file in the middle of the list, but adding
new `package.mill`s is pretty uncommon so we can probably fix that in a
follow up

Author: lihaoyi

integration/invalidation/codesig-subfolder/resources/subfolder2/package.mill

@@ -0,0 +1,5 @@
+package build.subfolder2
+import mill._
+
+
+def subFolder2Task = Task { println("running subFolder2Task") }

integration/invalidation/codesig-subfolder/resources/subfolder3/package.mill

@@ -0,0 +1,4 @@
+package build.subfolder3
+import mill._
+
+def subFolder2Task = Task { println("running subFolder3Task") }

integration/invalidation/codesig-subfolder/resources/subfolder4/package.mill

@@ -0,0 +1,4 @@
+package build.subfolder4
+import mill._
+
+def subFolder2Task = Task { println("running subFolder4Task") }

integration/invalidation/codesig-subfolder/resources/subfolder5/package.mill

@@ -0,0 +1,4 @@
+package build.subfolder5
+import mill._
+
+def subFolder2Task = Task { println("running subFolder3Task") }

integration/invalidation/codesig-subfolder/resources/subfolder6/package.mill

@@ -0,0 +1,4 @@
+package build.subfolder6
+import mill._
+
+def subFolder2Task = Task { println("running subFolder3Task") }

integration/invalidation/codesig-subfolder/resources/subfolder7/package.mill

@@ -0,0 +1,4 @@
+package build.subfolder7
+import mill._
+
+def subFolder2Task = Task { println("running subFolder3Task") }

integration/invalidation/codesig-subfolder/resources/subfolder8/package.mill

@@ -0,0 +1,4 @@
+package build.subfolder8
+import mill._
+
+def subFolder2Task = Task { println("running subFolder3Task") }

integration/invalidation/codesig-subfolder/resources/subfolder9/package.mill

@@ -0,0 +1,4 @@
+package build.subfolder9
+import mill._
+
+def subFolder2Task = Task { println("running subFolder3Task") }

integration/invalidation/codesig-subfolder/src/CodeSigSubfolderTests.scala

@@ -7,6 +7,9 @@ import utest._
 // Basically a copy of CodeSigHelloTests, but split across two files
 // (build.mill and subfolder/package.mill) and with some extra assertions
 // to exercise invalidation behavior specific to multi-file-builds
+//
+// Add a bunch of dummy subfolders to try and ensure that codesig is computed
+// correctly even in the presence of non-trivial subfolder `package.mill` setups
 object CodeSigSubfolderTests extends UtestIntegrationTestSuite {
   val tests: Tests = Tests {
     test("simple") - integrationTest { tester =>
@@ -15,7 +18,7 @@ object CodeSigSubfolderTests extends UtestIntegrationTestSuite {
       val initial = eval("foo")
 
       assert(initial.out.linesIterator.toSeq == Seq("running foo", "running helperFoo"))
-      assert(initial.err.contains("compiling 2 Scala sources"))
+      assert(initial.err.contains("compiling 10 Scala sources"))
 
       val cached = eval("foo")
       assert(cached.out == "")

runner/src/mill/runner/MillBuildRootModule.scala

@@ -46,8 +46,10 @@ abstract class MillBuildRootModule()(implicit
   def scriptSources: Target[Seq[PathRef]] = Task.Sources {
     MillBuildRootModule.parseBuildFiles(rootModuleInfo)
       .seenScripts
-      .keys.map(PathRef(_))
+      .keys
       .toSeq
+      .sorted // Ensure ordering is deterministic
+      .map(PathRef(_))
   }
 
   def parseBuildFiles: T[FileImportGraph] = Task {