cmd/go: `panic: LoadImport called with empty package path` when listing GOROOT/test/*.go [1.19 backport]

[gopherbot]

@matloob requested issue #60230 to be considered for backport to the next 1.19 minor release.

@gopherbot, please backport to 1.20. The change in go/parser behavior between Go 1.19 and Go 1.20 caused a regression in 1.20.

[matloob]

Whoops I didn't intend to create this