use ConcurrentHashMap to parallelize invocations with different tool …

…classpaths

src/main/scala/scalafix/internal/sbt/BlockingCache.scala

@@ -7,8 +7,7 @@ import scala.util.Try
 /** A basic thread-safe cache without any eviction. */
 class BlockingCache[K, V] {
 
- // Number of keys is expected to be very small so the global lock should not be a bottleneck
- private val underlying = ju.Collections.synchronizedMap(new ju.HashMap[K, V])
+ private val underlying = new ju.concurrent.ConcurrentHashMap[K, V]
 
  // Evaluations of `transform` are guaranteed to be sequential for the same key
  def compute(key: K, transform: Option[V] => Option[V]): V = {