cmd/go: mod tidy should ignore missing standard library packages [1.15 backport]

[gopherbot]

@jayconrod requested issue #44557 (fixed in 56d52e6 for Go 1.17) to be considered for backport to the next 1.15 minor release.

@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.

[dmitshur]

Approving per discussion in a release meeting. This backport applies to both 1.16 (#44793) and 1.15 (this issue).

[gopherbot]

Change https://golang.org/cl/298950 mentions this issue: [release-branch.go1.15] cmd/go: don't report missing std import errors for tidy and vendor

[gopherbot]

Closed by merging ba3dc70 to release-branch.go1.15.