Fixing single function deploys (pertaining to this feature, they are still broken from upstream however)

Author: Farley

lib/pip.js

@@ -304,11 +304,9 @@ function installAllRequirements() {
             this.servicePath, f.module, this.options, this.serverless
           );
           // Add symlinks into .serverless for each module so it's easier for injecting and for users to see where reqs are
-          if (reqsInstalledAt) {
-            fse.symlink(
-              reqsInstalledAt,
-              path.join(this.servicePath, '.serverless', `${f.module}_requirements`)
-            );
+          let symlinkPath = path.join(this.servicePath, '.serverless', `${f.module}_requirements`);
+          if (reqsInstalledAt && !fse.existsSync(symlinkPath)) {
+            fse.symlink( reqsInstalledAt, symlinkPath );
           }
           doneModules.push(f.module);
         }
@@ -318,8 +316,9 @@ function installAllRequirements() {
       this.servicePath, '', this.options, this.serverless
     );
     // Add symlinks into .serverless for so it's easier for injecting and for users to see where reqs are
-    if (reqsInstalledAt) {
-      fse.symlink(reqsInstalledAt, path.join(this.servicePath, '.serverless', `_requirements`));
+    let symlinkPath = path.join(this.servicePath, '.serverless', `_requirements`);
+    if (reqsInstalledAt && !fse.existsSync(symlinkPath)) {
+      fse.symlink(reqsInstalledAt, symlinkPath);
     }
   }
 };