Allow + in repo names (#1856)

**What type of PR is this?**

Bug fix

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

label

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

This accommodates the flip of `--incompatible_use_plus_in_repo_names` in
Bazel 8.

label/label.go

@@ -64,7 +64,8 @@ var NoLabel = Label{}
 
 var (
 	// This was taken from https://github.com/bazelbuild/bazel/blob/71fb1e4188b01e582a308cfe4bcbf1c730eded1b/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryName.java#L159C1-L164
-	labelRepoRegexp = regexp.MustCompile(`^@$|^[A-Za-z0-9_.-][A-Za-z0-9_.~-]*$`)
+	// ~ and + are both allowed as the former is used in canonical repo names by Bazel 7 and earlier and the latter in Bazel 8.
+	labelRepoRegexp = regexp.MustCompile(`^@$|^[A-Za-z0-9_.-][A-Za-z0-9_.~+-]*$`)
 	// This was taken from https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/cmdline/LabelValidator.java
 	// Package names may contain all 7-bit ASCII characters except:
 	// 0-31 (control characters)

label/label_test.go

@@ -88,6 +88,9 @@ func TestParse(t *testing.T) {
 		{str: "@rules_python~0.0.0~pip~name_dep//:_pkg", want: Label{Repo: "rules_python~0.0.0~pip~name_dep", Name: "_pkg"}},
 		{str: "@rules_python~0.0.0~pip~name//:dep_pkg", want: Label{Repo: "rules_python~0.0.0~pip~name", Name: "dep_pkg"}},
 		{str: "@@rules_python~0.26.0~python~python_3_10_x86_64-unknown-linux-gnu//:python_runtimes", want: Label{Repo: "rules_python~0.26.0~python~python_3_10_x86_64-unknown-linux-gnu", Name: "python_runtimes"}},
+		{str: "@rules_python++pip+name_dep//:_pkg", want: Label{Repo: "rules_python++pip+name_dep", Name: "_pkg"}},
+		{str: "@rules_python++pip+name//:dep_pkg", want: Label{Repo: "rules_python++pip+name", Name: "dep_pkg"}},
+		{str: "@@rules_python++python+python_3_10_x86_64-unknown-linux-gnu//:python_runtimes", want: Label{Repo: "rules_python++python+python_3_10_x86_64-unknown-linux-gnu", Name: "python_runtimes"}},
 	} {
 		got, err := Parse(tc.str)
 		if err != nil && !tc.wantErr {