Cleaned up SSH deployer's manager.

This PR cleans up the SSH deployer's manager. Specifically, I ported the
changes we made to the 'weaver multi' babysitter in ServiceWeaver#172 and ServiceWeaver#174. These
cleanups will make it easier to make some of the deployer API changes
we've been working on.

I noticed that the SSH babysitter re-uses a lot of the same code as the
'weaver multi' babysitter. I don't think we should merge the two into
one mega babysitter. In fact, I recently split the 'weaver multi'
babysitter from the weavertest babysitter, so I think there's value in
having things separate. But, maybe we can pull out some of the common
code without having a frankenstein babysitter API.

godeps.txt

@@ -572,7 +572,7 @@ github.com/ServiceWeaver/weaver/internal/tool/ssh/impl
  github.com/google/uuid
  go.opentelemetry.io/otel/sdk/trace
  golang.org/x/exp/maps
- google.golang.org/protobuf/proto
+ golang.org/x/exp/slices
  google.golang.org/protobuf/reflect/protoreflect
  google.golang.org/protobuf/runtime/protoimpl
  google.golang.org/protobuf/types/known/timestamppb
@@ -581,7 +581,6 @@ github.com/ServiceWeaver/weaver/internal/tool/ssh/impl
  net/http
  os
  os/exec
- path
  path/filepath
  reflect
  sort
@@ -602,12 +601,7 @@ github.com/ServiceWeaver/weaver/internal/traceio
  sync
  time
 github.com/ServiceWeaver/weaver/internal/versioned
- context
- github.com/ServiceWeaver/weaver/internal/cond
- github.com/ServiceWeaver/weaver/runtime/protomsg
  github.com/google/uuid
- google.golang.org/protobuf/proto
- strconv
  sync
 github.com/ServiceWeaver/weaver/metrics
  github.com/ServiceWeaver/weaver/runtime/metrics