cmd/dist: distpack-generated module files can't be downloaded with go mod download

[heschi]

While trying to release 1.21rc1 we hit this error from proxy.golang.org:

not found: unzip /tmp/gopath/pkg/mod/cache/download/golang.org/toolchain/@v/v0.0.1-go1.21rc1.linux-amd64.zip: golang.org/toolchain@v0.0.1-go1.21rc1.linux-amd64/src/cmd/go.mod: go.mod file not in module root directory
golang.org/toolchain@v0.0.1-go1.21rc1.linux-amd64/src/cmd/objdump/testdata/testfilenum/go.mod: go.mod file not in module root directory
golang.org/toolchain@v0.0.1-go1.21rc1.linux-amd64/src/crypto/internal/bigmod/_asm/go.mod: go.mod file not in module root directory
golang.org/toolchain@v0.0.1-go1.21rc1.linux-amd64/src/crypto/internal/edwards25519/field/_asm/go.mod: go.mod file not in module root directory
golang.org/toolchain@v0.0.1-go1.21rc1.linux-amd64/src/go.mod: go.mod file not in module root directory

dl2mod renamed go.mod files, but distpack doesn't.

cc @rsc

[gopherbot]

Change https://go.dev/cl/503978 mentions this issue: cmd/go: disable sumdb less often for toolchain downloads

[gopherbot]

Change https://go.dev/cl/503979 mentions this issue: cmd/distpack: rename go.mod to _go.mod in toolchain modules

[gopherbot]

Change https://go.dev/cl/504529 mentions this issue: internal/relui: add check that toolchain module works via GOTOOLCHAIN

[gopherbot]

Change https://go.dev/cl/504528 mentions this issue: internal/relui: skip waiting on proxy for Go 1.21 RC 1