Move convert_wycheproof into its own package

This can still be run like go run ../../util/convert_wycheproof. This is
part of an attempt to reland 54b04fdc21,
which ran into an issue with internal tooling that could not handle
standalone Go files.

Since the only such target we actually needed to run in that repository
is convert_wycheproof, just promote it into its own package. Then we can
stop trying to import util.

Change-Id: I6237777dad09e5c81ad961816ce14a287ab2d46a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/59185
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>

util/convert_wycheproof/convert_wycheproof.go

@@ -12,8 +12,8 @@
 // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
-// convert_wycheproof.go converts Wycheproof test vectors into a format more
-// easily consumed by BoringSSL.
+// convert_wycheproof converts Wycheproof test vectors into a format more easily
+// consumed by BoringSSL.
 package main
 
 import (