If you use -coverpkg=all you get coverage for all packages in the build.
Go 1.9 used a global counter for all the GoCover variables, so that they
were distinct for the entire build. The global counter caused problems
with caching, so we switched to a per-package counter. But now the
GoCover_0 in one package may be dot-imported into another and
conflict with the GoCover_0 in that other package.

Reestablish (overwhelmingly likely) global uniqueness of GoCover
variables by appending an _xxxxxxxxxxxx suffix, where the x's are
the prefix of the SHA256 hash of the import path. The point is only
to avoid accidents, not to defeat people determined to break the tools.

Fixes #23432.

Change-Id: I3088eceebbe35174f2eefe8d558b7c8b59d3eeac
Reviewed-on: https://go-review.googlesource.com/89135Reviewed-by: Ian Lance Taylor <iant@golang.org>