internal/fetch: close directory file in defer in extractReadme

This addresses bcmills's comment in CL 559615

Fixes golang/go#65545.

Change-Id: I02a4ba48eb897f33b4f4c47911e9cc80c99f7869
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/562176
TryBot-Result: Gopher Robot <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
kokoro-CI: kokoro <[email protected]>
Run-TryBot: Michael Matloob <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: matloob

internal/fetch/readme.go

@@ -35,6 +35,12 @@ func extractReadme(modulePath, dir, resolvedVersion string, contentDir fs.FS) (_
 		}
 		return nil, err
 	}
+	defer func() {
+		cerr := f.Close()
+		if err == nil {
+			err = cerr
+		}
+	}()
 	rdf, ok := f.(fs.ReadDirFile)
 	if !ok {
 		return nil, fmt.Errorf("could not open directory for %v", dir)