Feature: allow ignoring unused modules for go mod tidy

[annarosenthal]

Fixes golang/go#51262

This change modifies go mod tidy to allow ignoring unused imports by specifying an end-of-line comment containing ignore-unused.

Example:

module mod.com
go 1.14
require example.com v1.0.0
require ignored.com v1.0.0 // ignore-unused

In this example, go mod tidy will remove example.com and will keep ignored.com.

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