builder/remotecontext: deprecate CachableSource, NewCachableSource

This type (as well as TarsumBackup), was used for the experimental --stream
support for the classic builder. This feature was removed in commit
6ca3ec88ae9e1435abbed665ec598c00058659da, which also removed uses of
the CachableSource type.

As far as I could find, there's no external consumers of these types,
but let's deprecated it, to give potential users a heads-up that it
will be removed.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/remotecontext/tarsum.go

@@ -15,7 +15,9 @@ type hashed interface {
 	Digest() digest.Digest
 }
 
-// CachableSource is a source that contains cache records for its contents
+// CachableSource is a source that contains cache records for its contents.
+//
+// Deprecated: this type was used for the experimental "stream" support for the classic builder, which is no longer supported.
 type CachableSource struct {
 	mu   sync.Mutex
 	root string
@@ -23,7 +25,9 @@ type CachableSource struct {
 	txn  *iradix.Txn
 }
 
-// NewCachableSource creates new CachableSource
+// NewCachableSource creates new CachableSource.
+//
+// Deprecated: this type was used for the experimental "stream" support for the classic builder, which is no longer supported.
 func NewCachableSource(root string) *CachableSource {
 	ts := &CachableSource{
 		tree: iradix.New(),