cmd/go: accept @hash/branch in mod download

Go get in mod-enabled packages lets you do go get "pkg@<hash>" or "pkg@<branch>".
Go internally will switch the hash or branch into a pseudo version.
Go mod download should do the same. The bug lied in the fact that the disk cache
was not being written when Go converted the hash/branch into a pseduo version.

Fixes golang#27947

Change-Id: Ib5b376c65406173c3cd8739254b37a919dff8e8f

src/cmd/go/internal/modfetch/cache.go

@@ -129,16 +129,18 @@ func (r *cachingRepo) Stat(rev string) (*RevInfo, error) {
 		}
 		info, err = r.r.Stat(rev)
 		if err == nil {
-			if err := writeDiskStat(file, info); err != nil {
-				fmt.Fprintf(os.Stderr, "go: writing stat cache: %v\n", err)
-			}
 			// If we resolved, say, 1234abcde to v0.0.0-20180604122334-1234abcdef78,
 			// then save the information under the proper version, for future use.
 			if info.Version != rev {
+				file, _, _ = readDiskStat(r.path, info.Version)
 				r.cache.Do("stat:"+info.Version, func() interface{} {
 					return cachedInfo{info, err}
 				})
 			}
+
+			if err := writeDiskStat(file, info); err != nil {
+				fmt.Fprintf(os.Stderr, "go: writing stat cache: %v\n", err)
+			}
 		}
 		return cachedInfo{info, err}
 	}).(cachedInfo)