Make go_deps restarts less costly

internal/bzlmod/go_deps.bzl

@@ -13,7 +13,12 @@
 # limitations under the License.
 
 load("//internal:go_repository.bzl", "go_repository")
-load(":go_mod.bzl", "deps_from_go_mod", "sums_from_go_mod")
+load(
+    ":go_mod.bzl", 
+    "deps_from_go_mod",
+    "prefetch_files",
+    "sums_from_go_mod",
+)
 load(
     ":default_gazelle_overrides.bzl",
     "DEFAULT_BUILD_EXTRA_ARGS_BY_PATH",
@@ -191,6 +196,10 @@ def _noop(_):
     pass
 
 def _go_deps_impl(module_ctx):
+    for module in module_ctx.modules:
+        for from_file_tag in module.tags.from_file:
+            prefetch_files(module_ctx, from_file_tag.go_mod)
+
     module_resolutions = {}
     sums = {}
     replace_map = {}

internal/bzlmod/go_mod.bzl

@@ -16,6 +16,12 @@ visibility([
     "//tests/bzlmod/...",
 ])
 
+def prefetch_files(module_ctx, go_mod_label):
+    go_mod_path = module_ctx.path(go_mod_label)
+    go_sum_path = module_ctx.path(go_mod_path.dirname.get_child("go.sum"))
+    if go_sum_path.exists:
+        module_ctx.path(_get_go_sum_label(go_mod_label))
+
 def deps_from_go_mod(module_ctx, go_mod_label):
     """Loads the entries from a go.mod file.
 
@@ -225,13 +231,8 @@ def sums_from_go_mod(module_ctx, go_mod_label):
     _check_go_mod_name(go_mod_label.name)
 
     # We go through a Label so that the module extension is restarted if go.sum
-    # changes. We have to use a canonical label as we may not have visibility
-    # into the module that provides the go.sum.
-    go_sum_label = Label("@@{}//{}:{}".format(
-        go_mod_label.workspace_name,
-        go_mod_label.package,
-        "go.sum",
-    ))
+    # changes.
+    go_sum_label = _get_go_sum_label(go_mod_label)
     go_sum_content = module_ctx.read(go_sum_label)
     return parse_go_sum(go_sum_content)
 
@@ -252,3 +253,10 @@ def _canonicalize_raw_version(raw_version):
     if raw_version.startswith("v"):
         return raw_version[1:]
     return raw_version
+
+def _get_go_sum_label(go_mod_label):
+    return Label("@@{}//{}:{}".format(
+        go_mod_label.workspace_name,
+        go_mod_label.package,
+        "go.sum",
+    ))