Generate matrix nested manifests (#3431)

Generate matrix script finds manifests in subdirs

### Motivation:

The genereate matrix script attempts to derive the minimum Swift version
supported by the package so that it doesn't run CI on versions which are
older than that supported. At the moment it can't cope with repositories
with packages in subdirectories like swift-nio-examples.

### Modifications:

The script looks for manifests in subdirectories if
`FIND_SUBDIRECTORY_MANIFESTS_ENABLED=true`. This is opt-in because by
default CI only uses the top-level manifests.

### Result:

CI should generate more appropriate jobs.

An example of this in action
https://github.com/apple/swift-nio/actions/runs/18968739126/job/54171197381?pr=3431

Author: rnro

scripts/generate_matrix.sh

@@ -26,6 +26,8 @@ HEAD_BIN="${HEAD_BIN:-$(which head 2> /dev/null)}"; test -n "$HEAD_BIN" || fatal
 
 
 # Parameters
+find_subdirectory_manifests_enabled="${FIND_SUBDIRECTORY_MANIFESTS_ENABLED:=false}"
+
 linux_command="${MATRIX_LINUX_COMMAND:-}"  # required if any Linux pipeline is enabled
 linux_setup_command="${MATRIX_LINUX_SETUP_COMMAND:-}"
 linux_5_9_enabled="${MATRIX_LINUX_5_9_ENABLED:=false}"
@@ -124,20 +126,38 @@ version_gte() {
 }
 
 # Find minimum Swift version across all Package manifests
-# Checks Package.swift and all Package@swift-*.swift files
+# Checks Package.swift and all Package@swift-*.swift files in current directory and subdirectories
 # Returns the minimum tools version found across all manifests
 find_minimum_swift_version() {
     local min_version=""
 
-    # Check default Package.swift
+    # Check default Package.swift in current directory
     local default_version
     default_version=$(get_tools_version "Package.swift")
     if [[ -n "$default_version" ]]; then
         min_version="$default_version"
         log "Found Package.swift with tools-version: $default_version"
     fi
 
-    # Check all version-specific manifests
+    # Check all Package.swift files in subdirectories (multi-package repository support)
+    if [[ "$find_subdirectory_manifests_enabled" == "true" ]]; then
+        while read -r manifest; do
+            if [[ -f "$manifest" ]]; then
+                local version
+                version=$(get_tools_version "$manifest")
+                if [[ -n "$version" ]]; then
+                    log "Found $manifest with tools-version: $version"
+
+                    # If this version is less than current minimum, update minimum
+                    if [[ -z "$min_version" ]] || version_gte "$min_version" "$version"; then
+                        min_version="$version"
+                    fi
+                fi
+            fi
+        done < <(ls -1 ./*/Package.swift 2>/dev/null || true)
+    fi
+
+    # Check all version-specific manifests in current directory
     for manifest in Package@swift-*.swift; do
         if [[ ! -f "$manifest" ]]; then
             continue
@@ -157,6 +177,24 @@ find_minimum_swift_version() {
         fi
     done
 
+    # Check all version-specific manifests in subdirectories
+    if [[ "$find_subdirectory_manifests_enabled" == "true" ]]; then
+        while read -r manifest; do
+            if [[ -f "$manifest" ]]; then
+                local version
+                version=$(get_tools_version "$manifest")
+                if [[ -n "$version" ]]; then
+                    log "Found $manifest with tools-version: $version"
+
+                    # If this version is less than current minimum, update minimum
+                    if [[ -z "$min_version" ]] || version_gte "$min_version" "$version"; then
+                        min_version="$version"
+                    fi
+                fi
+            fi
+        done < <(ls -1 ./*/Package@swift-*.swift 2>/dev/null || true)
+    fi
+
     if [[ -n "$min_version" ]]; then
         echo "$min_version"
     else