modules: Ignore non-require blocks in go.mod rewrite

The Go 1.24 tool directive uses single-token entries inside a
tool ( ... ) block. The previous splitter treated any tab-indented
line as a require entry, causing an index out of range panic when
running hugo mod tidy on a module with a tool block.

Track the require block state explicitly so other blocks (tool,
replace, exclude, retract) are left untouched.

Fixes #14783

testscripts/commands/mod_tidy.txt

@@ -11,9 +11,17 @@ go 1.19
 
 require github.com/bep/empty-hugo-module v1.0.0
 
+tool (
+    github.com/gohugoio/hugo
+)
+
 module github.com/gohugoio/testmod
 -- golden/go.mod.cleaned --
 go 1.19
 
 
+tool (
+    github.com/gohugoio/hugo
+)
+
 module github.com/gohugoio/testmod