x/pkgsite: examples don't properly run for preview release modules

[codyoss]

What did you do?

Went to run the WithLabels example.

What did you expect to see?

A runnable example.

What did you see instead?

The following error in output:

go: finding module for package cloud.google.com/go/logging/jsonlog
go: downloading cloud.google.com/go/logging v1.4.2
go: downloading cloud.google.com/go v0.84.0
prog.go:4:2: module cloud.google.com/go/logging@latest found (v1.4.2), but does not contain package cloud.google.com/go/logging/jsonlog

Additional Context

• This is likely to to this being based off a pre-release tag: cloud.google.com/go/logging@v1.5.0-jsonlog-preview.
• This example does not actually produce any meaningful output, just noticed the error when I had click on it.

[jamalc]

We could disable or hide the run button for examples are not runnable or show a message in the output box that says as much. What do you think @julieqiu?

[julieqiu]

Both make sense to me.

[ZekeLu]

https://go.dev/cl/444455 for #50769 should have fixed this issue too.