exp/types: generalized GCImporter API.

exp/types: generalized GCImporter API.

- Renamed ExportData -> FindGcExportData
  and base it on an a bufio.Reader rather
  than a filename so it can be used in
  environments where object files are
  stored elsewhere.

- Factor former GcImporter into GcImportData
  and GcImport. Implementations with different
  storage locations for object files can build
  a customized GcImport using GcImportData.

This is pkg/exp only - no impact on Go 1.

R=golang-dev, lvd, rsc
CC=golang-dev
https://golang.org/cl/5574069