Fix incorrect comment parser used in nightly scaladoc (#18523)

Previously a `sourceDirectory` sbt task would resolve to
`library/src/main` which would usually be correct, but in the case of
scala3-library, we have the sources directly under `library/src`. This
lead to the fact that the incorrectly set `-comment-syntax` option could
never match against the prefix for markdown, and would use the wiki
syntax for all scala3-library sources.
Fixes #18511

Author: nicolasstucki

project/Build.scala

@@ -2207,8 +2207,7 @@ object ScaladocConfigs {
     val stdLibRoot = projectRoot.relativize(managedSources.toPath.normalize())
     val docRootFile = stdLibRoot.resolve("rootdoc.txt")
 
-    val dottyManagesSources =
-      (`scala3-library-bootstrapped`/Compile/sourceDirectory).value
+    val dottyManagesSources = (`scala3-library-bootstrapped`/Compile/baseDirectory).value
 
     val tastyCoreSources = projectRoot.relativize((`tasty-core-bootstrapped`/Compile/scalaSource).value.toPath().normalize())