all: Go 1.13 support

[dmitshur]

Now that Go 1.13 is out, gddo should be updated for it. This involves:

• [x] Re-generate the standard library list in package gosrc. Edit: Sent CL 193257.
• [x] Deploy with Go 1.13.
  • https://godoc.org/net/http#Request.Clone should show documentation for the Clone method of Request struct (it's new to 1.13).
  • https://godoc.org/internal/reflectlite should display package docs, rather than 404.

Previously #603 for Go 1.12.

/cc @andybons @shantuo @julieqiu

[dmitshur]

This is done now.