cmd/go: panic when argument doesn't match any packages [1.10 backport]

[gopherbot]

@odeke-em requested issue #28696 to be considered for backport to the next 1.10 minor release.

Thanks for reporting this @nicksnyder and for working on it @mark-rushakoff!

This is a new panic in Go1.11.*. On Go1.10, it printed out

$ go run nonexistent/...
go run: no go files listed

and now on Go.1.11, it panics as reported

$ go run nonexistent/...
go: warning: "nonexistent/..." matched no packages
panic: runtime error: index out of range

goroutine 1 [running]:
cmd/go/internal/run.runRun(0x195d2e0, 0xc0000ae0b0, 0x1, 0x1)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/go/internal/run/run.go:88 +0xc01
main.main()
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/cmd/go/main.go:219 +0x7d4

@gopherbot please open a backport tracking issue for 1.11.

[odeke-em]

For some reason Gopherbot opened up a backport issue for Go1.10.6 yet this problem only exists on Go1.11. I'll close this, please feel free to reopen if need be.