Fix a bug with unlimited parallel downloads

Previously no download workers are started when `--max-parallel-downloads`
is set to 0 which causes a crash. Now when this option is 0, a number of
workers equal to the number of packages in the lock file are started.

Related to #127

src/nimble.nim

@@ -554,7 +554,9 @@ proc processLockedDependencies(pkgInfo: PackageInfo, options: Options):
   downloadResults[].setLen(dependenciesToDownload[].len)
 
   var downloadWorkers: seq[Future[void]]
-  for i in 0 ..< options.maxParallelDownloads:
+  let workersCount = min(
+    options.maxParallelDownloads, dependenciesToDownload[].len)
+  for i in 0 ..< workersCount:
     downloadWorkers.add startDownloadWorker(
       dependenciesToDownload, options, downloadResults)
   waitFor all(downloadWorkers)

src/nimblepkg/options.nim

@@ -465,7 +465,10 @@ proc parseFlag*(flag, val: string, result: var Options, kind = cmdLongOption) =
   of "localdeps", "l": result.localdeps = true
   of "nosslcheck": result.disableSslCertCheck = true
   of "no-tarballs", "t": result.noTarballs = true
-  of "max-parallel-downloads", "m": result.maxParallelDownloads = parseInt(val)
+  of "max-parallel-downloads", "m":
+    result.maxParallelDownloads = parseInt(val)
+    if result.maxParallelDownloads == 0:
+      result.maxParallelDownloads = int.high
   else: isGlobalFlag = false
 
   var wasFlagHandled = true