x/playground: if Imports is checked, run 'go mod tidy' when the Format button is pressed

[bcmills]

CL 177421 added formatting for go.mod files in the Playground.

However, that isn't really enough to result in a reproducible build: as noted in #31944 (comment), unresolved dependencies are re-resolved whenever the cache is invalidated, which is difficult to predict.

One simple fix to add stability would be to have the existing Imports checkbox trigger a go mod tidy after running goimports. That would make the versions in use much clearer in the code and more stable over time, without the need for humans to manually resolve and copy over their go.mod files.

CC @dmitshur @ysmolsky

[bcmills]

This would also make it possible for users to add a go.sum file to their Playground example to show exactly which modules end up being loaded during the build. That seems like a handy bonus feature!

[dmitshur]

Also /cc @toothrot.

[toothrot]

It looks like the current implementation of Format uses the go/format core library instead of the Run approach of running exec.Command.

In order to run go mod tidy, the Format handler may need to act more like Run: writing the files to a temporary space, running a command, then in this case, reading the files back out.