cmd/go: support new hybrid coverage instrumentation
If GOEXPERIMENT=coverageredesign is in effect, introduce a new
top-level '-cover' option to "go build" to turn on new-style hybrid
code coverage instrumentation. Similarly, use the new instrumentation
for "go test -cover".
The main effects of "-cover" under the hood are to instrument files at
the package level using cmd/cover and to pass additional options to
the compiler when building instrumented packages.
The previous workflow for "go tool -cover mypkg" would expand to a
series of "go tool cover" commands (one per file) followed by a single
package compilation command to build the rewritten sources.
With the new workflow, the Go command will pass all of the Go files in
a package to the cover tool as a chunk (along with a config file
containing other parameters), then the cover tool will write
instrumented versions of the sources along with another "output"
config with info on coverage variable names for the the compiler. The
Go command will then kick off the compiler on the modified source
files, also passing in the config file generated by cmd/cover.