tcl.tclRequiresCheckHook: use structuredAttrs... structurally

Preserve the structure of tclRequiresCheck when using structuredAttrs
instead of relying on it being a space-separated string.
Shouldn't be necessary since package names don't usually contain spaces
or braces, but you never know.

pkgs/development/interpreters/tcl/tcl-requires-check-hook.sh

@@ -5,9 +5,28 @@ tclRequiresCheckPhase () {
     echo "Executing tclRequiresCheckPhase"
 
     if [ -n "$tclRequiresCheck" ]; then
-        echo "Check whether the following packages can be required: ${tclRequiresCheck[*]}"
         export TCLLIBPATH="$out/lib $TCLLIBPATH" # Redundant if tcl-package-hook is also used
-        tclRequiresCheck="${tclRequiresCheck[@]}" tclsh <<<'exit [catch {foreach req $env(tclRequiresCheck) {package require $req}}]'
+        tclsh <<'EOF'
+          if {[info exists env(NIX_ATTRS_JSON_FILE)]} {
+            set nix_attrs_json_file [open $env(NIX_ATTRS_JSON_FILE)]
+            set nix_attrs_json [read $nix_attrs_json_file]
+            close $nix_attrs_json_file
+            # Normally we'd use one of tcllib/tdom/rl_json/yajl-tcl to parse JSON,
+            # however we don't want to introduce a circular dependency,
+            # so we rely on the JSON capabilities of the builtin sqlite package
+            # https://wiki.tcl-lang.org/page/SQLite+extension+JSON1
+            package require sqlite3
+            sqlite3 db :memory: -readonly 1
+            set packages_to_check [db eval {
+              select value from json_each(jsonb_extract($nix_attrs_json, '$.tclRequiresCheck'))
+            }]
+            db close
+          } else {
+            set packages_to_check $env(tclRequiresCheck)
+          }
+          puts "Check whether the following packages can be required: $packages_to_check"
+          exit [catch {foreach pkg $packages_to_check {package require $pkg}}]
+EOF
     fi
 }