fix: replace moby/moby namesgenerator with internal implementation (#21377)

Replace the external moby/moby/pkg/namesgenerator dependency with an
internal implementation using gofakeit/v7. The moby package has ~25k
unique name combinations, and with its retry parameter only adds a
random digit 0-9, giving ~250k possibilities. In parallel tests, this
has led to collisions (flakes).

The new internal API at coderd/util/namesgenerator eliminates the
external dependnecy and offers functions with explicit uniqueness
guarantees. This PR also consolidates fragmented name generation in a
few places to use the new package.

| Old (moby/moby)                     | New                    |
|-------------------------------------|------------------------|
| namesgenerator.GetRandomName(0)     | NameWith("_")          |
| namesgenerator.GetRandomName(>0)    | NameDigitWith("_")     |
| testutil.GetRandomName(t)           | UniqueName()           |
| testutil.GetRandomNameHyphenated(t) | UniqueNameWith("-")    |

namesgenerator package API:
- NameWith(delim): random name, not unique
- NameDigitWith(delim): random name with 1-9 suffix, not unique
- UniqueName(): guaranteed unique via atomic counter
- UniqueNameWith(delim): unique with custom delimiter

Names continue to be docker style `[adjective][delim][surname]`. Unique
names are truncated to 32 characters (preserving the numeric suffix) to
fit common name length limits in Coder.

Related test flakes:
https://github.com/coder/internal/issues/1212
https://github.com/coder/internal/issues/118
https://github.com/coder/internal/issues/1068

coderd/userauth.go

@@ -19,7 +19,6 @@ import (
 	"github.com/go-jose/go-jose/v4/jwt"
 	"github.com/google/go-github/v43/github"
 	"github.com/google/uuid"
-	"github.com/moby/moby/pkg/namesgenerator"
 	"golang.org/x/oauth2"
 	"golang.org/x/xerrors"
 
@@ -41,6 +40,7 @@ import (
 	"github.com/coder/coder/v2/coderd/render"
 	"github.com/coder/coder/v2/coderd/telemetry"
 	"github.com/coder/coder/v2/coderd/userpassword"
+	"github.com/coder/coder/v2/coderd/util/namesgenerator"
 	"github.com/coder/coder/v2/coderd/util/ptr"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/cryptorand"
@@ -1723,7 +1723,7 @@ func (api *API) oauthLogin(r *http.Request, params *oauthLoginParams) ([]*http.C
 					validUsername bool
 				)
 				for i := 0; i < 10; i++ {
-					alternate := fmt.Sprintf("%s-%s", original, namesgenerator.GetRandomName(1))
+					alternate := fmt.Sprintf("%s-%s", original, namesgenerator.NameDigitWith("_"))
 
 					params.Username = codersdk.UsernameFrom(alternate)