go/ast: deprecate MergePackageFiles (it's not used, and buggy)

Breakingstdlib

What changed: go/ast.MergePackageFiles is now deprecated because it produces broken merges when parser.ParseComments is enabled.
Production impact: The source does not say.
Try it: Search your code for uses of MergePackageFiles and replace them with alternative parsing logic.
Source: github.com/golang/go/issues/7124

Written by gpt-oss-20b from the linked source · claims checked against the sources · archive, not individually reviewed