x/tools/gopls: support all flags regardless of their position

[stamblerre]

For the gopls command line, some flags must be passed directly to the gopls command and some flags must be passed to serve. This makes it difficult for editor clients to start gopls using the gopls serve command and generally leads to confusion. @ianthehat has work in progress to allow flags to be specified in any order. This is a prerequisite to #41282.