introduce std.Build.Cache.Manifest.addFilePath

and deprecate `addFile`. Part of an effort to move towards using
`std.Build.Cache.Path` abstraction in more places, which makes it easier
to avoid absolute paths and path resolution.

lib/std/Build/Cache.zig

@@ -354,6 +354,19 @@ pub const Manifest = struct {
     /// ```
     /// var file_contents = cache_hash.files.keys()[file_index].contents.?;
     /// ```
+    pub fn addFilePath(m: *Manifest, file_path: Path, max_file_size: ?usize) !usize {
+        const gpa = m.cache.gpa;
+        try m.files.ensureUnusedCapacity(gpa, 1);
+        const resolved_path = try fs.path.resolve(gpa, &.{
+            file_path.root_dir.path orelse ".",
+            file_path.subPathOrDot(),
+        });
+        errdefer gpa.free(resolved_path);
+        const prefixed_path = try m.cache.findPrefixResolved(resolved_path);
+        return addFileInner(m, prefixed_path, max_file_size);
+    }
+
+    /// Deprecated; use `addFilePath`.
     pub fn addFile(self: *Manifest, file_path: []const u8, max_file_size: ?usize) !usize {
         assert(self.manifest_file == null);
 
@@ -362,6 +375,10 @@ pub const Manifest = struct {
         const prefixed_path = try self.cache.findPrefix(file_path);
         errdefer gpa.free(prefixed_path.sub_path);
 
+        return addFileInner(self, prefixed_path, max_file_size);
+    }
+
+    fn addFileInner(self: *Manifest, prefixed_path: PrefixedPath, max_file_size: ?usize) !usize {
         const gop = self.files.getOrPutAssumeCapacityAdapted(prefixed_path, FilesAdapter{});
         if (gop.found_existing) {
             gop.key_ptr.updateMaxSize(max_file_size);