internal/exportdata: introduce shared library for exportdata
Deduplicates FindPkg and FindExportData which were shared by
go/internal/gcimporter and cmd/compile/internal/importer into
a new package internal/exportdata.
This change only moves code.
Change-Id: I1daf24dd79fafbe9014b2b15671dcde46b54711e
Reviewed-on: https://go-review.googlesource.com/c/go/+/626700
Commit-Queue: Tim King <taking@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>