Leakage of apiserver dependencies into api package

Importing the `github.com/juju/juju/api` client package adds lots of additional dependencies. Some of which are leaked from the `apiserver` package. Try the reproducer below:

```
#! /bin/sh -e

rm -fr test-juju
mkdir test-juju
cd test-juju

go mod init test-juju

# Add simple main file that instantiates a charm client.
cat <<EOF > main.go
package main

import (
 "github.com/juju/juju/api/client/charms"
 "github.com/juju/juju/api/connector"
)

func main() {
 connector, _ := connector.NewSimple(connector.SimpleConfig{})
 conn, _ := connector.Connect()
 charms.NewClient(conn)
}
EOF

cat <<EOF >> go.mod
// Exclude some modules that cause an ambiguous import failure.
exclude (
 cloud.google.com/go v0.26.0
 cloud.google.com/go v0.65.0
 cloud.google.com/go v0.81.0
)
EOF

go mod tidy

# Some non-API packages used:
go mod why github.com/juju/juju/apiserver
go mod why github.com/juju/juju/state
go mod why github.com/juju/juju/caas

# AWS SDK is imported (I'm not sure this is intentional and should be included in client code).
go mod why github.com/aws/aws-sdk-go-v2
```