cmd/go: all pattern in per package flags is inaccurate in module mode

[matloob]

all has a slightly different meaning in module mode than it does in GOPATH mode. In GOPATH mode, it's pretty much everything:

In GOPATH mode, "all" expands to all packages found in all the GOPATH trees.

But in module mode, "all"

expands to the set of "packages in the main module (or workspace modules) and their dependencies, including dependencies needed by tests of any of those"

according to go help packages.

We don't seem to handle this correctly when processing "all" in -coverpkg and the per package flags -asmflags, -gccgoflags, -gcflags, and -ldflags. The cmd/go/internal/load.MatchPackage function just matches every package for all.