Use file tags to generate conversions

This drives conversion generation from file tags like:
  // +conversion-gen=k8s.io/my/internal/version
.. rather than hardcoded lists of packages.

The only net change in generated code can be explained as correct.  Previously
it didn't know that conversion was available.

pkg/api/v1/generated.proto

@@ -2481,7 +2481,7 @@ message Secret {
   // It is provided as a write-only convenience method.
   // All keys and values are merged into the data field on write, overwriting any existing values.
   // It is never output when reading from the API.
-  // +genconversion=false
+  // +k8s:conversion-gen=false
   map<string, string> stringData = 4;
 
   // Used to facilitate programmatic handling of secret data.
@@ -2715,7 +2715,7 @@ message ServiceSpec {
   // API for compatibility until at least 8/20/2016.  It will be removed from
   // any new API revisions.  If both deprecatedPublicIPs *and* externalIPs are
   // set, deprecatedPublicIPs is used.
-  // +genconversion=false
+  // +k8s:conversion-gen=false
   repeated string deprecatedPublicIPs = 6;
 
   // Supports "ClientIP" and "None". Used to maintain session affinity.