replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/golang.org/x/text/unicode/bidi/bidi.go

@@ -6,7 +6,7 @@
 
 // Package bidi contains functionality for bidirectional text support.
 //
-// See http://www.unicode.org/reports/tr9.
+// See https://www.unicode.org/reports/tr9.
 //
 // NOTE: UNDER CONSTRUCTION. This API may change in backwards incompatible ways
 // and without notice.