[HCL2] Allow to override top-level source-block params in build

#### Description

I want to reuse Packer HCL source-blocks as templates. In such source-blocks I want to define default values in parameters and have the ability override this parameters in the build blocks.

Example:

```hcl
source "qemu" "my-image" {
disk_size = "10G"
...
}

build build {
source "qemu.my-image" {
disk_size = "30G"
name = "build1"
}

source "qemu.my-image" {
disk_size = "25G"
name = "build2"
}
...
```

But Packer currently does not allow duplicate and override such params (`disk_size` in example above). See Packer [docs](https://developer.hashicorp.com/packer/docs/templates/hcl_templates/blocks/build/source).

#### Use Case(s)

Reusing Packer source-blocks and creation "library" of HCL-templates.