go.tools/go/gccgoimporter: keep track of package and import priority

Clients such as compilers need this information in order
to correctly link against imported packages.

This also adds support for the condensed import data format
where the priority information is stored as a suffix of the
condensed import data, as well as support for archive files.

LGTM=gri
R=gri
CC=golang-codereviews, iant
https://golang.org/cl/78740043
11 files changed