Add fetchLocalFirst, fix in the way local repo are handled

cli/src/main/scala/coursier/cli/Helper.scala

@@ -192,7 +192,7 @@ class Helper(
     filter = Some(dep => keepOptional || !dep.optional)
   )
 
-  val fetchQuiet = coursier.fetch(repositories)
+  val fetchQuiet = coursier.fetchLocalFirst(repositories)
   val fetch0 =
     if (verbose0 == 0) fetchQuiet
     else {

core/shared/src/main/scala/coursier/package.scala

@@ -1,3 +1,4 @@
+import scalaz.{ -\/, \/- }
 import scalaz.concurrent.Task
 
 /**
@@ -109,4 +110,27 @@ package object coursier {
       )
   }
 
+  def fetchLocalFirst(
+    repositories: Seq[core.Repository]
+  )(implicit
+    cachePolicy: CachePolicy
+  ): ResolutionProcess.Fetch[Task] = {
+
+    modVers =>
+      Task.gatherUnordered(
+        modVers
+          .map {case (module, version) =>
+            def attempt(cachePolicy: CachePolicy) =
+              Repository.find(repositories, module, version)(cachePolicy)
+                .run
+                .map((module, version) -> _)
+
+            attempt(CachePolicy.LocalOnly).flatMap {
+              case v @ (_, \/-(_)) => Task.now(v)
+              case (_, -\/(_)) => attempt(cachePolicy)
+            }
+          }
+      )
+  }
+
 }

files/src/main/scala/coursier/Files.scala

@@ -38,7 +38,7 @@ case class Files(
             cacheDir + "/" + url.stripPrefix(base)
         }
 
-    if (artifact.extra.contains("local") || isLocal)
+    if (artifact.extra.contains("local"))
       artifact
     else
       artifact.copy(extra = artifact.extra + ("local" ->
@@ -141,14 +141,19 @@ case class Files(
 
 
     val tasks =
-      for ((f, url) <- pairs if url != ("file:" + f) && url != ("file://" + f)) yield {
-        val file = new File(f)
-        cachePolicy[FileError \/ File](
-          _.isLeft )(
-          locally(file) )(
-          _ => remote(file, url)
-        ).map(e => (file, url) -> e.map(_ => ()))
-      }
+      for ((f, url) <- pairs) yield
+        if (url != ("file:" + f) && url != ("file://" + f)) {
+          assert(!f.startsWith("file:/"), s"Wrong file detection: $f, $url")
+          val file = new File(f)
+          cachePolicy[FileError \/ File](
+            _.isLeft)(
+            locally(file))(
+            _ => remote(file, url)
+          ).map(e => (file, url) -> e.map(_ => ()))
+        } else {
+          val file = new File(f)
+          Task.now(((file, url), \/-(())))
+        }
 
     Nondeterminism[Task].gather(tasks)
   }