Automatically deleting corrupted Go module cache (#2194)

**What type of PR is this?**

Feature


**What package or component does this PR mostly affect?**
go_repository

**What does this PR do? Why is it needed?**
When the Go module sum mismatches, the most effective way is to delete
the corrupted Go module, instead of deleting the whole Go module cache,
which may contain many other unrelated modules. Because fetch_repo
already know where the Go module is, it can automatically delete it and
ask people to retry.

**Other notes for review**

New experience:
```
$ export GO_REPOSITORY_USE_HOST_CACHE=1
# intentionally corrupt a module
$ rm $(go env GOMODCACHE)/github.com/mattn/[email protected]/static_mock.go
$ bazel clean --expunge --async
$ bazel fetch --force @com_github_mattn_go_sqlite3//:go-sqlite3
...
ERROR: com_github_mattn_go_sqlite3: fetch_repo: resulting module with sum h1:NlqjV/4w2ZBKwamqKNXcb6I12S9NTFOi18B0VlNju6g=; expected sum h1:vfoHhTN1af61xCRSWzFIWzx2YskyMTwHLrExkBOjvxI=. Please try again. If the problem persists, please try clearing your host module cache with `go clean -modcache`
```

Author: linzhp

cmd/fetch_repo/module.go

@@ -16,6 +16,7 @@ limitations under the License.
 package main
 
 import (
+	"errors"
 	"fmt"
 	"os"
 
@@ -65,10 +66,19 @@ func fetchModule(dest, importpath, version, sum string) error {
 	}
 
 	if repoSum != sum {
+		errMsg := fmt.Sprintf("resulting module with sum %s; expected sum %s.", repoSum, sum)
+		// The module cache is corrupt. Remove the downloaded directory.
+		if err := os.RemoveAll(dl.Dir); err != nil {
+			errMsg += fmt.Sprintf(" Additionally, failed to remove corrupt module cache directory %q: %v. Please remove it manaully and retry.", dl.Dir, err)
+		} else {
+			errMsg += " Please retry."
+		}
 		if goModCache := os.Getenv("GOMODCACHE"); goModCache != "" {
-			return fmt.Errorf("resulting module with sum %s; expected sum %s, Please try clearing your module cache directory %q", repoSum, sum, goModCache)
+			errMsg += fmt.Sprintf(" If the problem persists, please try clearing your host module cache with `go clean -modcache`")
+		} else {
+			errMsg += fmt.Sprintf(" If the problem persists, please try clearing Bazel output directory with `bazel clean --expunge`")
 		}
-		return fmt.Errorf("resulting module with sum %s; expected sum %s", repoSum, sum)
+		return errors.New(errMsg)
 	}
 
 	return nil