x/tools/gopls: use a per-process shared temp directory

[findleyr]

gopls can generate a fairly wide-range of temp files and directories (logs, heap dumps, temp modfiles, gc details data, and now temp workspace directories).

We should consolidate these into a top-level temp directory for the gopls process, both so that they are co-located, and so that we can have a catch-all clean up pass when the gopls process exits.

[heschi]

I think this is generally good, but we want the memory debug zips to be discoverable. IMO either the temp dirs should be predictably named, or the debug zips should stay at the top level.

[findleyr]

I was imagining /tmp/gopls-<pid>/, which could contain trace.log, stderr.log, details/, workspace/, 1GB-nonames.zip, for example.

[gopherbot]

Change https://golang.org/cl/300409 mentions this issue: internal/lsp: add a temp workspace per folder, and a helper command