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>

tstest/integration/tailscaled_deps_test_darwin.go

@@ -38,7 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tailfs"
+	_ "tailscale.com/tailfs/tailfsimpl"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -38,7 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tailfs"
+	_ "tailscale.com/tailfs/tailfsimpl"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_linux.go

@@ -38,7 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tailfs"
+	_ "tailscale.com/tailfs/tailfsimpl"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -38,7 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tailfs"
+	_ "tailscale.com/tailfs/tailfsimpl"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_windows.go

@@ -45,7 +45,7 @@ import (
 	_ "tailscale.com/safesocket"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tailfs"
+	_ "tailscale.com/tailfs/tailfsimpl"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"