Fix a buglet in cmd/cover in how we handle package name/path for the
"go build -o foo.exe *.go" and "go run *.go" cases.
The go command assigns a dummy import path of "command-line-arguments"
to the main package built in these cases; rather than expose this
dummy to the user in coverage reports, the cover tool had a special
case hack intended to rewrite such package paths to "main". The hack
was too general, however, and was rewriting the import path of all
packages with (p.name == "main") to an import path of "main". The hack
also produced unexpected results for cases such as
go test -cover foo.go foo_test.go
This patch removes the hack entirely, leaving the package path for
such cases as "command-line-arguments".