Don't prefer Cask files in the cwd

Fixes #4641.  Cask files in the cwd are still permitted, but
at the very lowest priority, below Tapped Cask names.

USAGE.md

@@ -221,12 +221,15 @@ above, a Cask name on the command line can take the form of:
 * a Cask name as returned by `brew cask search`, _eg_: `google-chrome`
 * a fully-qualified Cask name which includes the Tap, _eg_: `caskroom/fonts/font-symbola`
 
-`brew cask` also accepts two other forms for Cask names:
+`brew cask` also accepts three other forms for Cask names:
 
-* a fully-qualified pathname to a Cask file, _eg_: `/usr/local/Cellar/brew-cask/0.25.0/Casks/google-chrome.rb`
+* a path to a Cask file, _eg_: `/usr/local/Cellar/brew-cask/0.25.0/Casks/google-chrome.rb`
 * a `curl`-retrievable URI to a Cask file, _eg_: `https://raw.github.com/caskroom/homebrew-cask/f54bbfaae0f2fa7210484f46313a459cb8a14d2f/Casks/google-chrome.rb`
+* a file in the current working directory, _eg_: `my-modfied-google-chrome.rb`.  Note
+  that Tapped Casks names will be preferred over this form.  To force the use of a Cask
+  file in the current directory, specify a pathname with slashes, _eg_: `./google-chrome.rb`.
 
-The last two forms are intended for users who wish to maintain private Casks.
+The last three forms are intended for users who wish to maintain private Casks.
 
 ## Taps
 

lib/cask/source.rb

@@ -1,7 +1,8 @@
 module Cask::Source; end
 
 require 'cask/source/gone'
-require 'cask/source/path'
+require 'cask/source/path_slash_required'
+require 'cask/source/path_slash_optional'
 require 'cask/source/tapped_qualified'
 require 'cask/source/untapped_qualified'
 require 'cask/source/tapped'
@@ -11,10 +12,11 @@ module Cask::Source
   def self.sources
     [
       Cask::Source::URI,
-      Cask::Source::Path,
+      Cask::Source::PathSlashRequired,
       Cask::Source::TappedQualified,
       Cask::Source::UntappedQualified,
       Cask::Source::Tapped,
+      Cask::Source::PathSlashOptional,
       Cask::Source::Gone,
     ]
   end

lib/cask/source/path.rb

@@ -1,26 +0,0 @@
-class Cask::Source::Path
-  def self.me?(query)
-    query_with_ext = "#{query}"
-    query_with_ext.concat('.rb') unless query_with_ext.match(%r{\.rb\Z}i)
-    # bug? arguably, we should not pick up a relative path
-    # containing a Cask file so easily, since this source
-    # is tested before the default Tap.  Perhaps there
-    # should be two Path sources, absolute and relative.
-    File.file?(query_with_ext)
-  end
-
-  attr_reader :path
-
-  def initialize(path)
-    @path = Pathname(path).expand_path
-  end
-
-  def load
-    require path
-    Cask.const_get(cask_class_name).new
-  end
-
-  def cask_class_name
-    path.basename.to_s.sub(/\.rb/, '').split('-').map(&:capitalize).join
-  end
-end

lib/cask/source/path_base.rb

@@ -0,0 +1,25 @@
+class Cask::Source::PathBase
+
+  # derived classes must define method self.me?
+
+  def self.path_for_query(query)
+    path_string = "#{query}"
+    path_string.concat('.rb') unless path_string.match(%r{\.rb\Z}i)
+    Pathname.new(path_string)
+  end
+
+  attr_reader :path
+
+  def initialize(path)
+    @path = Pathname(path).expand_path
+  end
+
+  def load
+    require path
+    Cask.const_get(cask_class_name).new
+  end
+
+  def cask_class_name
+    path.basename.to_s.sub(/\.rb/, '').split('-').map(&:capitalize).join
+  end
+end

lib/cask/source/path_slash_optional.rb

@@ -0,0 +1,8 @@
+require 'cask/source/path_base'
+
+class Cask::Source::PathSlashOptional < Cask::Source::PathBase
+  def self.me?(query)
+    path = self.path_for_query(query)
+    path.exist?
+  end
+end

lib/cask/source/path_slash_required.rb

@@ -0,0 +1,8 @@
+require 'cask/source/path_base'
+
+class Cask::Source::PathSlashRequired < Cask::Source::PathBase
+  def self.me?(query)
+    path = self.path_for_query(query)
+    path.to_s.include?('/') and path.exist?
+  end
+end

lib/cask/source/tapped.rb

@@ -25,6 +25,6 @@ class Cask::Source::Tapped
 
   def load
     path = self.class.path_for_query(title)
-    Cask::Source::Path.new(path).load
+    Cask::Source::PathSlashOptional.new(path).load
   end
 end

lib/cask/source/uri.rb

@@ -15,7 +15,7 @@ class Cask::Source::URI
     ohai "Downloading #{uri}"
     odebug "Download target -> #{path.to_s}"
     curl(uri, '-o', path.to_s)
-    Cask::Source::Path.new(path).load
+    Cask::Source::PathSlashOptional.new(path).load
   rescue ErrorDuringExecution
     raise CaskUnavailableError, uri
   end