Enhance develop --with-dependencies

Make `develop --with-dependencies` to skip already existing directories
when cloning the repositories. This is useful when a second
`develop --with-dependencies` is executed for another package and some
of the dependencies are the same as in the first run. In that case, we
don't want the entire command to fail because some package directories
already exist.

Related to nim-lang#127

src/nimble.nim

@@ -537,7 +537,19 @@ proc downloadDependency(name: string, dep: LockFileDep, options: Options):
     getDevelopDownloadDir(url, subdir, options) else: ""
 
   if dirExists(downloadPath):
-    raiseCannotCloneInExistingDirException(downloadPath)
+    if options.developWithDependencies:
+      displayWarning(skipDownloadingInAlreadyExistingDirectoryMsg(
+        downloadPath, name))
+      result = DownloadInfo(
+        name: name,
+        dependency: dep,
+        url: url,
+        version: version,
+        downloadDir: downloadPath,
+        vcsRevision: dep.vcsRevision.newClone)
+      return
+    else:
+      raiseCannotCloneInExistingDirException(downloadPath)
 
   let (downloadDir, _, vcsRevision) = await downloadPkg(
     url, version, dep.downloadMethod, subdir, options, downloadPath,
@@ -1213,7 +1225,16 @@ proc installDevelopPackage(pkgTup: PkgTuple, options: var Options):
   let downloadDir = getDevelopDownloadDir(url, subdir, options)
 
   if dirExists(downloadDir):
-    raiseCannotCloneInExistingDirException(downloadDir)
+    if options.developWithDependencies:
+      displayWarning(skipDownloadingInAlreadyExistingDirectoryMsg(
+        downloadDir, pkgTup.name))
+      let pkgInfo = getPkgInfo(downloadDir, options)
+      developFromDir(pkgInfo, options)
+      options.action.devActions.add(
+        (datAdd, pkgInfo.getNimbleFileDir.normalizedPath))
+      return pkgInfo
+    else:
+      raiseCannotCloneInExistingDirException(downloadDir)
 
   # Download the HEAD and make sure the full history is downloaded.
   let ver =

src/nimblepkg/displaymessages.nim

@@ -153,3 +153,7 @@ proc pkgAlreadyExistsInTheCacheMsg*(pkgInfo: PackageInfo): string =
      pkgInfo.basicInfo.name,
     $pkgInfo.basicInfo.version,
     $pkgInfo.basicInfo.checksum)
+
+proc skipDownloadingInAlreadyExistingDirectoryMsg*(dir, name: string): string =
+  &"The download directory \"{dir}\" already exists.\n" &
+  &"Skipping the download of \"{name}\"."