x/tools/gopls: redundant diagnostics for initialization cycle

[findleyr]

In the following code, gopls produces multiple diagnostics at X:

var X = Y

var Y = Z

var Z = X

This is due to the way gopls unpacks the initialization cycle errors returned by go/types.
Minor, but filing this for tracking purposes. If/when we improve structured errors in go/types, we should fix this. We could also add some ad-hoc suppression of the duplicates.

[gopherbot]

Change https://go.dev/cl/566075 mentions this issue: gopls/internal/test/marker: add a test for initialization cycle errors