cmd/go: add 'go generate -skip'

[xtonyjiang]

We currently have the -run flag, but I believe it is currently impossible to supply a regular expression that would allow for excluding a certain expression (due to no lookaheads in Go's regex syntax). Being able to exclude expressions would be useful if we want to exclude certain directives that are time-consuming and are not needed in the moment, and we want to run a large amount of other directives, so that it would be impractical to simply run go generate multiple times with different -run flags.

I think one way to implement this could be an additional flag. Similar to grep's --invert-match flag, we could do something like:

go generate -invertmatch -run="my excluded pattern" ./...

Alternatively, we could also have a different flag that is used instead of -run, like:

go generate -runexcept="my excluded pattern" ./...

Here, supplying both -runexcept and -run would be invalid.

[rsc]

I'm not sure we should do anything here, but if we did, it should apply to the other -run flags too.

[jayconrod]

#41583 is an accepted proposal for a go test -skip flag that sounds similar. What about bringing that to go generate?

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

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