go/packages: 1.10 compatibility: support absolute path patterns

[matloob]

go/packages should support providing absolute paths to packages in 1.10. It does in 1.11, so this is a backwards-compatibility bug.

Also see https://gophers.slack.com/archives/C0VPK4Z5E/p1536879551000100

[gopherbot]

Change https://golang.org/cl/137096 mentions this issue: go/packages: allow absolute paths when using the fallback