fetch: clear partial downloads when --force is given

Fixes Homebrew/homebrew#23764.

Library/Homebrew/cmd/fetch.rb

@@ -46,7 +46,7 @@ module Homebrew extend self
   private
 
   def fetch_fetchable f
-    f.cached_download.rmtree if already_fetched?(f) && ARGV.force?
+    f.clear_cache if ARGV.force?
     download = f.fetch
 
     return unless download.file?

Library/Homebrew/download_strategy.rb

@@ -35,6 +35,7 @@ class AbstractDownloadStrategy
   def fetch; end
   def stage; end
   def cached_location; end
+  def clear_cache; end
 end
 
 class VCSDownloadStrategy < AbstractDownloadStrategy
@@ -63,6 +64,10 @@ class VCSDownloadStrategy < AbstractDownloadStrategy
   def cached_location
     @clone
   end
+
+  def clear_cache
+    cached_location.rmtree if cached_location.exist?
+  end
 end
 
 class CurlDownloadStrategy < AbstractDownloadStrategy
@@ -86,6 +91,10 @@ class CurlDownloadStrategy < AbstractDownloadStrategy
     tarball_path
   end
 
+  def clear_cache
+    [cached_location, temporary_path].each { |f| f.unlink if f.exist? }
+  end
+
   def downloaded_size
     temporary_path.size? or 0
   end

Library/Homebrew/formula.rb

@@ -192,6 +192,10 @@ class Formula
     downloader.cached_location
   end
 
+  def clear_cache
+    downloader.clear_cache
+  end
+
   # Can be overridden to selectively disable bottles from formulae.
   # Defaults to true so overridden version does not have to check if bottles
   # are supported.

Library/Homebrew/resource.rb

@@ -43,6 +43,10 @@ class Resource
     downloader.cached_location
   end
 
+  def clear_cache
+    downloader.clear_cache
+  end
+
   # Fetch, verify, and unpack the resource
   def stage(target=nil, &block)
     verify_download_integrity(fetch)