cmd/go: 'go mod edit' should validate the provided version format

[hyangah]

Before editing the contents, the newly added values need validation.
Currently the error checking is done only when parsing the existing go.mod contents.

In the following commands sequence, the error in the first go mod edit command is not reported until the next go mod command.

$ go mod init example.com/mod
go: creating new go.mod: module example.com/mod
$ go mod edit -require golang.org/x/text/cases@latest
$ go mod edit -require golang.org/x/text/date@latest 
go: errors parsing go.mod:
/scratch/mod/go.mod:3: invalid module version "latest": version must be of the form v1.2.3

[andybons]

@rsc @bcmills

[rsc]

Sorry, but working as intended and documented. Edit is for editing, primarily to be invoked by tools. Most people should not run edit directly. Instead, if you want to add a requirement with all the semantic extras, use 'go get'.

'go help mod edit' explains:

The -require=path@version and -droprequire=path flags ....
These flags are mainly for tools that understand the module graph.
Users should prefer 'go get path@version' or 'go get path@none',
which make other go.mod adjustments as needed to satisfy
constraints imposed by other modules.

[hyangah]

"go get" currently involves checking module lists over network. If change to the module is not committed upstream yet (e.g., in the middle of local development), we need a way to configure the require+replace rules and 'go get' doesn't support the use case. I observed multiple users (including me) suggested hand-edit of go.mod files with fake versions as a workaround. That's not ideal and we need a reliable tool (if go mod edit -require is not the recommended path).

[myitcv]

I think the loop here is closed by #26241