[Analyzer] Ensure external sources are downloaded when needed

Closes https://github.com/CocoaPods/CocoaPods/issues/2494.

CHANGELOG.md

@@ -16,6 +16,11 @@ To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides
   [Samuel Giddins](https://github.com/segiddins)
   [#2624](https://github.com/CocoaPods/CocoaPods/issues/2624)
 
+* Ensure that external sources (as specified in the `Podfile`) are downloaded
+  when their source is missing, even if their specification is present.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#2494](https://github.com/CocoaPods/CocoaPods/issues/2494)
+
 
 ## 0.34.2
 

lib/cocoapods/installer/analyzer.rb

@@ -283,7 +283,7 @@ module Pod
           end
           deps_to_fetch = deps_with_external_source.select { |dep| pods_to_fetch.include?(dep.root_name) }
           deps_to_fetch_if_needed = deps_with_external_source.select { |dep| result.podfile_state.unchanged.include?(dep.root_name) }
-          deps_to_fetch += deps_to_fetch_if_needed.select { |dep| sandbox.specification(dep.root_name).nil? || !dep.external_source[:local].nil? || !dep.external_source[:path].nil? }
+          deps_to_fetch += deps_to_fetch_if_needed.select { |dep| sandbox.specification(dep.root_name).nil? || !dep.external_source[:local].nil? || !dep.external_source[:path].nil? || !sandbox.pod_dir(dep.root_name).directory? }
         end
 
         unless deps_to_fetch.empty?