audit: formally ban go get usage

Library/Homebrew/dev-cmd/audit.rb

@@ -885,6 +885,10 @@ def audit_text
       problem "Formulae using virtualenvs do not need a `setuptools` resource."
     end
 
+    if text =~ /system\s+['"]go['"],\s+['"]get['"]/
+      problem "Formulae should not use `go get`. If non-vendored resources are required use `go_resource`s."
+    end
+
     return unless text.include?('require "language/go"') && !text.include?("go_resource")
     problem "require \"language/go\" is unnecessary unless using `go_resource`s"
   end

Library/Homebrew/test/dev-cmd/audit_spec.rb

@@ -430,6 +430,21 @@ def install
       expect(fa.problems.first)
         .to match('xcodebuild should be passed an explicit "SYMROOT"')
     end
+
+    specify "disallow go get usage" do
+      fa = formula_auditor "foo", <<-EOS.undent
+        class Foo <Formula
+          url "http://example.com/foo-1.0.tgz"
+
+          def install
+            system "go", "get", "bar"
+          end
+        end
+      EOS
+      fa.audit_text
+      expect(fa.problems.first)
+        .to match("Formulae should not use `go get`. If non-vendored resources are required use `go_resource`s.")
+    end
   end
 
   describe "#audit_revision_and_version_scheme" do