cmd/go: do not report cached libraries as stale in module mode

[bcmills]

In module mode, non-main packages have no install path: built packages are stored only in the build cache. In module mode, go install for a non-main package only ensures that the build cache is up to date; it does not install the library elsewhere.

According to https://tip.golang.org/cmd/go/#hdr-List_packages_or_modules, the Stale field means:

would go install do anything for this package?

According to that interpretation, we should not report a library as Stale in module mode if it is already up to date in the build cache. Today, we always report libraries as stale:

$ go version
go version devel go1.17-a9bb38222 Thu Jun 24 03:45:33 2021 +0000 linux/amd64

$ go mod init example.com/m
go: creating new go.mod: module example.com/m

$ go get -d golang.org/x/tools/imports
go: downloading golang.org/x/tools v0.1.4
go: downloading golang.org/x/mod v0.4.2
go: downloading golang.org/x/sys v0.0.0-20210510120138-977fb7262007
go: downloading golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
go get: added golang.org/x/mod v0.4.2
go get: added golang.org/x/sys v0.0.0-20210510120138-977fb7262007
go get: added golang.org/x/tools v0.1.4
go get: added golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1

$ go list -f '{{if .Stale}}{{.StaleReason}}{{end}}' golang.org/x/tools/imports
stale dependency: golang.org/x/tools/internal/event/label

$ go install golang.org/x/tools/imports

$ go list -f '{{if .Stale}}{{.StaleReason}}{{end}}' golang.org/x/tools/imports
not installed but available in build cache

$ go install golang.org/x/tools/imports

$ go list -f '{{if .Stale}}{{.StaleReason}}{{end}}' golang.org/x/tools/imports
not installed but available in build cache

This came to my attention via https://stackoverflow.com/q/64462190.

CC @jayconrod @matloob

[bcmills]

This appears to have been fixed incidentally by (or in association with) #47257.

$ go version
go version devel go1.20-888047c310 Sun Dec 11 16:34:38 2022 +0000 linux/amd64

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

$ go get -d golang.org/x/tools/imports
go: downloading golang.org/x/tools v0.4.0
go: downloading golang.org/x/mod v0.7.0
go: downloading golang.org/x/sys v0.3.0
go: added golang.org/x/mod v0.7.0
go: added golang.org/x/sys v0.3.0
go: added golang.org/x/tools v0.4.0

$ go list -f '{{if .Stale}}{{.StaleReason}}{{end}}' golang.org/x/tools/imports
stale dependency: internal/goarch

$ go install golang.org/x/tools/imports

$ go list -f '{{if .Stale}}{{.StaleReason}}{{end}}' golang.org/x/tools/imports