cmd/go: 'go mod edit' can add requirements that it can't remove

[bcmills]

go mod edit -require can be used to add versions that don't match the major version of the corresponding module.

Unfortunately, the major-version validation happens early during parsing, so go mod edit -droprequire cannot subsequently drop those requirements.

example.com$ gotip mod init example.com
go: creating new go.mod: module example.com

example.com$ gotip version
go version devel +9838f4df Fri Jun 14 18:41:07 2019 +0000 linux/amd64

example.com$ gotip mod edit -require golang.org/x/text@v2.0.0

example.com$ gotip mod edit -droprequire golang.org/x/text
go: errors parsing go.mod:
/tmp/tmp.5UPKq2KvZc/example.com/go.mod:5: invalid module: golang.org/x/text should be v0 or v1, not v2 (v2.0.0)

example.com$ cat go.mod
module example.com

go 1.13

require golang.org/x/text v2.0.0

example.com$

CC @jayconrod

See also #30513, #26454.

[hyangah]

FYI #27060 asked to validate the added requirement but was closed as working as intended.

[bcmills]

Yeah, I think the fix for this should go in the other direction: we should move the major-version validation from modfile.Parse to some later step.