cmd/go: `panic: LoadImport called with empty package path` when listing GOROOT/test/*.go [1.20 backport]

[gopherbot]

@matloob requested issue #60230 to be considered for backport to the next 1.20 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.

[gopherbot]

Change https://go.dev/cl/502697 mentions this issue: go/build: check for invalid import paths again

[gopherbot]

Closed by merging f5172dc to release-branch.go1.20.