Parse Mill build files (build.mill / build.sc)

[andrew]

Mill (https://github.com/com-lihaoyi/mill) is the second-most-common Scala build tool after sbt. internal/maven/sbt.go already parses build.sbt and sbt's dependencyDot output; Mill has no equivalent parser, so a Mill project's declared dependencies are invisible.

Mill declares dependencies as string literals inside build.mill (Mill 0.12+) or build.sc (older):

def ivyDeps = Seq(
  ivy"org.typelevel::cats-core:2.12.0",   // :: is Scala cross-versioned
  ivy"com.google.guava:guava:33.2.1-jre", // :  is a plain Maven coordinate
)

Mill 0.12+ also accepts mvn"..." as an alias for ivy"...". Both forms map to Maven coordinates: :: expands to groupId:artifactId_<scalaBinaryVersion>:version (the binary version can be left as-is in the raw name, matching how sbt.go handles %%), and single : is groupId:artifactId:version verbatim.

A regex over ivy"..." / mvn"..." literals would give declared-dependency coverage on par with the existing build.sbt parser. Registration:

core.Register("maven", core.Manifest, &millParser{}, core.ExactMatch("build.mill", "build.sc"))

Related: #77 (JVM row) tracks other Maven-ecosystem format additions.