added Atlas helper tool (#18497)

* added Atlas helper tool
* further improvements

tools/atlas/osutils.nim

@@ -0,0 +1,59 @@
+## OS utilities like 'withDir'.
+## (c) 2021 Andreas Rumpf
+
+import os, strutils, osproc
+
+template withDir*(dir, body) =
+  let oldDir = getCurrentDir()
+  try:
+    setCurrentDir(dir)
+    body
+  finally:
+    setCurrentDir(oldDir)
+
+proc isUrl*(x: string): bool =
+  x.startsWith("git://") or x.startsWith("https://") or x.startsWith("http://")
+
+proc cloneUrl*(url, dest: string; cloneUsingHttps: bool): string =
+  ## Returns an error message on error or else "".
+  result = ""
+  var modUrl =
+    if url.startsWith("git://") and cloneUsingHttps:
+      "https://" & url[6 .. ^1]
+    else: url
+
+  # github + https + trailing url slash causes a
+  # checkout/ls-remote to fail with Repository not found
+  var isGithub = false
+  if modUrl.contains("github.com") and modUrl.endswith("/"):
+    modUrl = modUrl[0 .. ^2]
+    isGithub = true
+
+  let (_, exitCode) = execCmdEx("git ls-remote --quiet --tags " & modUrl)
+  var xcode = exitCode
+  if isGithub and exitCode != QuitSuccess:
+    # retry multiple times to avoid annoying github timeouts:
+    for i in 0..4:
+      os.sleep(4000)
+      xcode = execCmdEx("git ls-remote --quiet --tags " & modUrl)[1]
+      if xcode == QuitSuccess: break
+
+  if xcode == QuitSuccess:
+    # retry multiple times to avoid annoying github timeouts:
+    let cmd = "git clone " & modUrl & " " & dest
+    for i in 0..4:
+      if execShellCmd(cmd) == 0: return ""
+      os.sleep(4000)
+    result = "exernal program failed: " & cmd
+  elif not isGithub:
+    let (_, exitCode) = execCmdEx("hg identify " & modUrl)
+    if exitCode == QuitSuccess:
+      let cmd = "hg clone " & modUrl & " " & dest
+      for i in 0..4:
+        if execShellCmd(cmd) == 0: return ""
+        os.sleep(4000)
+      result = "exernal program failed: " & cmd
+    else:
+      result = "Unable to identify url: " & modUrl
+  else:
+    result = "Unable to identify url: " & modUrl