tailfs: clean up naming and package structure

- Restyles tailfs -> tailFS
- Defines interfaces for main TailFS types
- Moves implemenatation of TailFS into tailfsimpl package

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailcfg/tailcfg.go

@@ -1345,8 +1345,8 @@ const (
 	// PeerCapabilityWebUI grants the ability for a peer to edit features from the
 	// device Web UI.
 	PeerCapabilityWebUI PeerCapability = "tailscale.com/cap/webui"
-	// PeerCapabilityTailfs grants the ability for a peer to access tailfs shares.
-	PeerCapabilityTailfs PeerCapability = "tailscale.com/cap/tailfs"
+	// PeerCapabilityTailFS grants the ability for a peer to access tailfs shares.
+	PeerCapabilityTailFS PeerCapability = "tailscale.com/cap/tailfs"
 )
 
 // NodeCapMap is a map of capabilities to their optional values. It is valid for
@@ -2211,8 +2211,8 @@ const (
 	// tail end of an active direct connection in magicsock.
 	NodeAttrProbeUDPLifetime NodeCapability = "probe-udp-lifetime"
 
-	// NodeAttrsTailfsSharingEnabled enables sharing via Tailfs.
-	NodeAttrsTailfsSharingEnabled NodeCapability = "tailfs:share"
+	// NodeAttrsTailFSSharingEnabled enables sharing via TailFS.
+	NodeAttrsTailFSSharingEnabled NodeCapability = "tailfs:share"
 )
 
 // SetDNSRequest is a request to add a DNS record.

tailcfg/tailcfg_test.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	. "tailscale.com/tailcfg"
+	"tailscale.com/tstest/deptest"
 	"tailscale.com/types/key"
 	"tailscale.com/types/opt"
 	"tailscale.com/types/ptr"
@@ -842,3 +843,14 @@ func TestRawMessage(t *testing.T) {
 		})
 	}
 }
+
+func TestDeps(t *testing.T) {
+	deptest.DepChecker{
+		BadDeps: map[string]string{
+			// Make sure we don't again accidentally bring in a dependency on
+			// TailFS or its transitive dependencies
+			"tailscale.com/tailfs/tailfsimpl": "https://github.com/tailscale/tailscale/pull/10631",
+			"github.com/tailscale/gowebdav":   "https://github.com/tailscale/tailscale/pull/10631",
+		},
+	}.Check(t)
+}