props @DirtyF: handle static files in a collection

Static files within a collection inside a custom collections_dir needs
to output to the same path as when the collection is at the root of the
source directory.

features/collections_dir.feature

@@ -144,6 +144,9 @@ Feature: Collections Directory
 
     collections_dir: gathering
     """
+    And I have a "gathering/_puppies/static_file.txt" file that contains "Static content."
+    And I have a gathering/_puppies/nested directory
+    And I have a "gathering/_puppies/nested/static_file.txt" file that contains "Nested Static content."
     When I run jekyll build --drafts
     Then I should get a zero exit status
     And the _site directory should exist
@@ -152,5 +155,7 @@ Feature: Collections Directory
     And the "_site/2009/03/27/draft-in-gathering.html" file should exist
     And the "_site/2009/03/27/draft-at-root.html" file should not exist
     And the "_site/puppies/rover-at-root.html" file should not exist
+    And I should see exactly "Static content." in "_site/puppies/static_file.txt"
+    And I should see exactly "Nested Static content." in "_site/puppies/nested/static_file.txt"
     And the _site/gathering directory should not exist
     And the _site/_posts directory should not exist

lib/jekyll/collection.rb

@@ -226,7 +226,8 @@ def read_static_file(file_path, full_path)
         site.source,
         relative_dir,
         File.basename(full_path),
-        self
+        self,
+        site.config["collections_dir"]
       )
     end
   end

lib/jekyll/static_file.rb

@@ -26,12 +26,13 @@ def reset_cache
     # dir  - The String path between <source> and the file.
     # name - The String filename of the file.
     # rubocop: disable ParameterLists
-    def initialize(site, base, dir, name, collection = nil)
+    def initialize(site, base, dir, name, collection = nil, collections_dir = "")
       @site = site
       @base = base
       @dir  = dir
       @name = name
       @collection = collection
+      @collections_dir = collections_dir
       @relative_path = File.join(*[@dir, @name].compact)
       @extname = File.extname(@name)
       @data = @site.frontmatter_defaults.all(relative_path, type)
@@ -40,7 +41,12 @@ def initialize(site, base, dir, name, collection = nil)
 
     # Returns source file path.
     def path
-      File.join(*[@base, @dir, @name].compact)
+      # Static file is from a collection inside custom collections directory
+      if !@collection.nil? && !@collections_dir.empty?
+        File.join(*[@base, @collections_dir, @dir, @name].compact)
+      else
+        File.join(*[@base, @dir, @name].compact)
+      end
     end
 
     # Obtain destination path.