Move SCM wrappers from Contributions to ENV/scm.

Closes Homebrew/homebrew#32615.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/ENV/4.3/git

@@ -1 +1 @@
-../../Contributions/cmd/git
+../scm/git

Library/ENV/4.3/svn

@@ -1 +1 @@
-../../Contributions/cmd/svn
+../scm/svn

Library/brew.rb

@@ -108,11 +108,13 @@ begin
   end
 
   # Add contributed commands to PATH before checking.
-  ENV['PATH'] += "#{File::PATH_SEPARATOR}#{HOMEBREW_CONTRIB}/cmd"
   Dir["#{HOMEBREW_LIBRARY}/Taps/*/*/cmd"].each do |tap_cmd_dir|
     ENV["PATH"] += "#{File::PATH_SEPARATOR}#{tap_cmd_dir}"
   end
 
+  # Add SCM wrappers.
+  ENV["PATH"] += "#{File::PATH_SEPARATOR}#{HOMEBREW_LIBRARY}/ENV/scm"
+
   internal_cmd = require? HOMEBREW_LIBRARY_PATH.join("cmd", cmd) if cmd
 
   # Usage instructions should be displayed if and only if one of: