Move sync generics to their own packages

The presence of multiple packages in a single directory sometimes
confuses `go mod`, producing output like:
  go: downloading gvisor.dev/gvisor v0.0.0-20210601174640-77dc0f5bc94d
  $GOMODCACHE/gvisor.dev/gvisor@v0.0.0-20210601174640-77dc0f5bc94d/pkg/linewriter/linewriter.go:21:2: found packages sync (aliases.go) and seqatomic (generic_atomicptr_unsafe.go) in $GOMODCACHE/gvisor.dev/gvisor@v0.0.0-20210601174640-77dc0f5bc94d/pkg/sync
  imports.go:67:2: found packages tcp (accept.go) and rcv (rcv_test.go) in $GOMODCACHE/gvisor.dev/gvisor@v0.0.0-20210601174640-77dc0f5bc94d/pkg/tcpip/transport/tcp

PiperOrigin-RevId: 376956213

pkg/sentry/kernel/BUILD

@@ -32,7 +32,7 @@ go_template_instance(
     out = "seqatomic_taskgoroutineschedinfo_unsafe.go",
     package = "kernel",
     suffix = "TaskGoroutineSchedInfo",
-    template = "//pkg/sync:generic_seqatomic",
+    template = "//pkg/sync/seqatomic:generic_seqatomic",
     types = {
         "Value": "TaskGoroutineSchedInfo",
     },

pkg/sentry/kernel/auth/BUILD

@@ -8,7 +8,7 @@ go_template_instance(
     out = "atomicptr_credentials_unsafe.go",
     package = "auth",
     suffix = "Credentials",
-    template = "//pkg/sync:generic_atomicptr",
+    template = "//pkg/sync/atomicptr:generic_atomicptr",
     types = {
         "Value": "Credentials",
     },

pkg/sentry/kernel/futex/BUILD

@@ -8,7 +8,7 @@ go_template_instance(
     out = "atomicptr_bucket_unsafe.go",
     package = "futex",
     suffix = "Bucket",
-    template = "//pkg/sync:generic_atomicptr",
+    template = "//pkg/sync/atomicptr:generic_atomicptr",
     types = {
         "Value": "bucket",
     },

pkg/sentry/time/BUILD

@@ -8,7 +8,7 @@ go_template_instance(
     out = "seqatomic_parameters_unsafe.go",
     package = "time",
     suffix = "Parameters",
-    template = "//pkg/sync:generic_seqatomic",
+    template = "//pkg/sync/seqatomic:generic_seqatomic",
     types = {
         "Value": "Parameters",
     },

pkg/sync/BUILD

@@ -1,5 +1,4 @@
 load("//tools:defs.bzl", "go_library", "go_test")
-load("//tools/go_generics:defs.bzl", "go_template")
 
 package(
     default_visibility = ["//:sandbox"],
@@ -8,45 +7,6 @@ package(
 
 exports_files(["LICENSE"])
 
-go_template(
-    name = "generic_atomicptr",
-    srcs = ["generic_atomicptr_unsafe.go"],
-    types = [
-        "Value",
-    ],
-)
-
-go_template(
-    name = "generic_atomicptrmap",
-    srcs = ["generic_atomicptrmap_unsafe.go"],
-    opt_consts = [
-        "ShardOrder",
-    ],
-    opt_types = [
-        "Hasher",
-    ],
-    types = [
-        "Key",
-        "Value",
-    ],
-    deps = [
-        ":sync",
-        "//pkg/gohacks",
-    ],
-)
-
-go_template(
-    name = "generic_seqatomic",
-    srcs = ["generic_seqatomic_unsafe.go"],
-    types = [
-        "Value",
-    ],
-    deps = [
-        ":sync",
-        "//pkg/gohacks",
-    ],
-)
-
 go_library(
     name = "sync",
     srcs = [

pkg/sync/atomicptr/BUILD

@@ -1,14 +1,23 @@
 load("//tools:defs.bzl", "go_library", "go_test")
-load("//tools/go_generics:defs.bzl", "go_template_instance")
+load("//tools/go_generics:defs.bzl", "go_template", "go_template_instance")
 
 package(licenses = ["notice"])
 
+go_template(
+    name = "generic_atomicptr",
+    srcs = ["generic_atomicptr_unsafe.go"],
+    types = [
+        "Value",
+    ],
+    visibility = ["//:sandbox"],
+)
+
 go_template_instance(
     name = "atomicptr_int",
     out = "atomicptr_int_unsafe.go",
     package = "atomicptr",
     suffix = "Int",
-    template = "//pkg/sync:generic_atomicptr",
+    template = ":generic_atomicptr",
     types = {
         "Value": "int",
     },

pkg/sync/atomicptrmap/BUILD

@@ -1,17 +1,36 @@
 load("//tools:defs.bzl", "go_library", "go_test")
-load("//tools/go_generics:defs.bzl", "go_template_instance")
+load("//tools/go_generics:defs.bzl", "go_template", "go_template_instance")
 
 package(
     default_visibility = ["//visibility:private"],
     licenses = ["notice"],
 )
 
+go_template(
+    name = "generic_atomicptrmap",
+    srcs = ["generic_atomicptrmap_unsafe.go"],
+    opt_consts = [
+        "ShardOrder",
+    ],
+    opt_types = [
+        "Hasher",
+    ],
+    types = [
+        "Key",
+        "Value",
+    ],
+    deps = [
+        "//pkg/gohacks",
+        "//pkg/sync",
+    ],
+)
+
 go_template_instance(
     name = "test_atomicptrmap",
     out = "test_atomicptrmap_unsafe.go",
     package = "atomicptrmap",
     prefix = "test",
-    template = "//pkg/sync:generic_atomicptrmap",
+    template = ":generic_atomicptrmap",
     types = {
         "Key": "int64",
         "Value": "testValue",
@@ -27,7 +46,7 @@ go_template_instance(
     package = "atomicptrmap",
     prefix = "test",
     suffix = "Sharded",
-    template = "//pkg/sync:generic_atomicptrmap",
+    template = ":generic_atomicptrmap",
     types = {
         "Key": "int64",
         "Value": "testValue",

pkg/sync/seqatomic/BUILD

@@ -1,14 +1,27 @@
 load("//tools:defs.bzl", "go_library", "go_test")
-load("//tools/go_generics:defs.bzl", "go_template_instance")
+load("//tools/go_generics:defs.bzl", "go_template", "go_template_instance")
 
 package(licenses = ["notice"])
 
+go_template(
+    name = "generic_seqatomic",
+    srcs = ["generic_seqatomic_unsafe.go"],
+    types = [
+        "Value",
+    ],
+    visibility = ["//:sandbox"],
+    deps = [
+        ":sync",
+        "//pkg/gohacks",
+    ],
+)
+
 go_template_instance(
     name = "seqatomic_int",
     out = "seqatomic_int_unsafe.go",
     package = "seqatomic",
     suffix = "Int",
-    template = "//pkg/sync:generic_seqatomic",
+    template = ":generic_seqatomic",
     types = {
         "Value": "int",
     },