internal/exportdata, cmd/compile/internal/noder: merge export data handling

Unify how go/types, types2, and noder read in unified export data from
GC-created files.

This splits FindExportData into smaller pieces for improved code
sharing.
- FindPackageDefinition finds the package definition file in the ar
  archive.
- ReadObjectHeaders reads the object headers.
- ReadExportDataHeader reads the export data format header.

There is a new convenience wrapper ReadUnified that combines all of
these. This documents the expected archive contents.

Updates noder and the importers to use these.
This also adjusts when end-of-section marker ("\n$$\n") checking happens.

Change-Id: Iec2179b0a1ae7f69eb12d077018f731116a77f13
Reviewed-on: https://go-review.googlesource.com/c/go/+/628155
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Tim King <taking@google.com>

0 files changed, 0 insertions, 0 deletions