Revert deletion of two not used procedures

The two currently not used procedures `getSpecificDir` and `doPull` from
`download.nim` file are reverted according to dom96's suggestion in the
code review of pull request #692. Now they are marked with used pragma
to disable the warning.

Related to #680

src/nimblepkg/download.nim

@@ -10,6 +10,13 @@ type
   DownloadMethod* {.pure.} = enum
     git = "git", hg = "hg"
 
+proc getSpecificDir(meth: DownloadMethod): string {.used.} =
+  case meth
+  of DownloadMethod.git:
+    ".git"
+  of DownloadMethod.hg:
+    ".hg"
+
 proc doCheckout(meth: DownloadMethod, downloadDir, branch: string) =
   case meth
   of DownloadMethod.git:
@@ -23,6 +30,19 @@ proc doCheckout(meth: DownloadMethod, downloadDir, branch: string) =
     cd downloadDir:
       doCmd("hg checkout " & branch)
 
+proc doPull(meth: DownloadMethod, downloadDir: string) {.used.} =
+  case meth
+  of DownloadMethod.git:
+    doCheckout(meth, downloadDir, "")
+    cd downloadDir:
+      doCmd("git pull")
+      if existsFile(".gitmodules"):
+        doCmd("git submodule update")
+  of DownloadMethod.hg:
+    doCheckout(meth, downloadDir, "default")
+    cd downloadDir:
+      doCmd("hg pull")
+
 proc doClone(meth: DownloadMethod, url, downloadDir: string, branch = "",
              onlyTip = true) =
   case meth