cmd/go: mod tidy should ignore missing standard library packages

[rsc]

We know the standard library will not have dependencies on other modules, so in this example it shouldn't matter that Go 1.15 cannot find io/fs:

% cd /Users/rsc/src/golang.org/x/tools
% go mod tidy
% go1.16 mod tidy
% go1.15 mod tidy
golang.org/x/tools/cmd/present imports
	embed: package embed is not in GOROOT (/Users/rsc/sdk/go1.15/src/embed)
golang.org/x/tools/cmd/present imports
	io/fs: package io/fs is not in GOROOT (/Users/rsc/sdk/go1.15/src/io/fs)
golang.org/x/tools/go/analysis/passes/buildtag imports
	go/build/constraint: package go/build/constraint is not in GOROOT (/Users/rsc/sdk/go1.15/src/go/build/constraint)
% 

That is, go1.15 mod tidy should work.

And we should think about backporting this change.

/cc @bcmills @jayconrod @matloob

[zikaeroh]

See also #40067 (which is this issue).

[gopherbot]

Change https://golang.org/cl/297489 mentions this issue: cmd/go: mod tidy ignores missing standard library packages

[gopherbot]

Change https://golang.org/cl/298749 mentions this issue: cmd/go: don't report missing std import errors for tidy and vendor

[jayconrod]

@gopherbot Please backport to 1.15 and 1.16. These errors are being reported more frequently since popular new packages were introduced in 1.16.

[gopherbot]

Backport issue(s) opened: #44792 (for 1.15), #44793 (for 1.16).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.