go mod download -json github.com/go-delve/delve@latest
shows a Dir that contains vendor/modules.txt.
None of the other vendor files are present, just modules.txt.

This is probably a bug: we intended for modules to contain
no vendored info at all, but we had to leave files in the root
alone because they might be for a package or command
named vendor (only subdirectories are trimmed).

We should probably update the logic to trim modules.txt
as well, but only when the go.mod has a new enough Go version.

/cc @bcmills