Support (and ignore) ‘tool’ directive in go.mod files. (#2064)

See https://go.dev/doc/modules/gomod-ref#tool and
https://go.dev/doc/go1.24#tools.

Fixes #2031

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**What type of PR is this?**

Feature

**What package or component does this PR mostly affect?**

all

**What does this PR do? Why is it needed?**

Don't fail parsing of go.mod if it contains a tool directive.

**Which issues(s) does this PR fix?**

Fixes #2031

Author: phst

internal/bzlmod/go_mod.bzl

@@ -210,7 +210,7 @@ def parse_go_mod(content, path):
             continue
 
         if not current_directive:
-            if tokens[0] not in ["module", "go", "require", "replace", "exclude", "retract", "toolchain"]:
+            if tokens[0] not in ["module", "go", "require", "replace", "exclude", "retract", "toolchain", "tool"]:
                 fail("{}:{}: unexpected token '{}' at start of line".format(path, line_no, tokens[0]))
             if len(tokens) == 1:
                 fail("{}:{}: expected another token after '{}'".format(path, line_no, tokens[0]))

tests/bzlmod/go_mod_test.bzl

@@ -25,6 +25,8 @@ module github.com/bazelbuild/bazel-gazelle
   retract v1.0.0
 
 require golang.org/x/sys v0.0.0-20220624220833-87e55d714810 // indirect
+
+tool  golang.org/x/tools/cmd/bisect
 """
 
 _EXPECTED_GO_MOD_PARSE_RESULT = struct(