Warn when incorrectly using replaces

This DSL line was introduced only to deal with the renaming of the "chef-full"
artifacts to "chef" back in the dark ages.  It has been copypasta'd around to
other projects to the point where there was a completely inaccurate @todo added
to make replaces default to a broken configuration. Very few projects should be
using replaces and including a replaces line which is the same as the
package_name will break RPM upgrades.

Patch validates that replaces != package_name and yells at the
user to knock if off if they're doing that.

lib/omnibus/exceptions.rb

@@ -82,6 +82,27 @@ def to_s
     end
   end
 
+  class BadReplacesLine < RuntimeError
+    def to_s
+      <<-EOH
+The `replaces` project DSL statement should never equal the `package_name` or
+`name` of a project. The `replaces` option should only be used when you have
+published an artifact under one name and then later renamed the packages that
+you are publishing and you must obsolete the old package name. For example this
+is used, correctly, in chef-client builds:
+
+    name 'chef'
+    replaces 'chef-full'
+
+The RPMs and debs which were originally published were named "chef-full" and
+this was later renamed, and in order to upgrade the (very old) "chef-full"
+packages the new "chef" packages needed to know to uninstall any "chef-full"
+packages that were found. Projects which have never been renamed, however,
+should never use the replaces line.
+      EOH
+    end
+  end
+
   class NoPackageFile < RuntimeError
     def initialize(package_path)
       @package_path = package_path

lib/omnibus/generator_files/project.rb.erb

@@ -3,7 +3,6 @@ name '<%= config[:name] %>'
 maintainer 'CHANGE ME'
 homepage 'CHANGEME.com'
 
-replaces        '<%= config[:name] %>'
 install_path    '<%= config[:install_path] %>'
 build_version   Omnibus::BuildVersion.new.semver
 build_iteration 1

lib/omnibus/project.rb

@@ -141,11 +141,15 @@ def package_me
 
     # Ensures that certain project information has been set
     #
-    # @raise [MissingProjectConfiguration] if a required parameter has
-    #   not been set
+    # @todo raise MissingProjectConfiguration instead of printing the warning
+    #   in the next major release
+    #
     # @return [void]
     def validate
       name && install_path && maintainer && homepage
+      if package_name == replaces
+        log.warn { BadReplacesLine.new.message }
+      end
     end
 
     # @!group DSL methods
@@ -300,11 +304,12 @@ def description(val = NULL_ARG)
     # Ultimately used as the value for the `--replaces` flag in
     # {https://github.com/jordansissel/fpm fpm}.
     #
+    # This should only be used when renaming a package and obsoleting the old
+    # name of the package.  Setting this to the same name as package_name will
+    # cause RPM upgrades to fail.
+    #
     # @param val [String] the name of the package to replace
     # @return [String]
-    #
-    # @todo Consider having this default to {#package_name}; many uses of this
-    #   method effectively do this already.
     def replaces(val = NULL_ARG)
       @replaces = val unless val.equal?(NULL_ARG)
       @replaces