x/tools/cmd/cover: isolate, tag, and delete package

[bcmills]

The documentation for x/tools/cmd/cover says:

Deprecated: For Go releases 1.5 and later, this tool lives in the standard repository. The code here is not maintained.

I count 11 CLs updating the package since that notice was first added in CL 9539.
Keeping the code around clearly isn't doing anyone any good — at the very least, it adds noise in git grep results (which is how I came to notice it in the first place).

I propose that we give it the same treatment as x/exp/winfsnotify: carve it out into a nested module, tag that module cmd/cover/v0.1.0-deprecated, and then delete it from the x/tools repo at HEAD.

[mvdan]

I wonder - is this trick a general solution to deleting code from stable modules? I guess it technically is a breaking change, but we're OK with that given that x/tools is a v0, and the workaround is easy - use the new module instead.

[gopherbot]

Change https://go.dev/cl/451595 mentions this issue: cmd/cover: carve out deprecated command into its own module

[gopherbot]

Change https://go.dev/cl/451596 mentions this issue: cmd/cover: delete package

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group