Getting Yul ASTs requires asking for ast at contract level instead of at file level

## Description

When compiling in Yul mode, including `"ast"` in the file-level output selection doesn't result in getting ASTs like it does when compiling Solidity. Instead you have to set it at the contract level. This makes no sense, because ASTs are a file-level matter, and is also inconsistent with how the compiler acts when compiling Solidity.

## Environment

- Compiler version: 0.8.21
- Target EVM version (as per compiler settings): default

## Steps to Reproduce

Really any Yul code will do here! What's relevant here is the output selection. An `outputSelection` of

```
{
"*": {
"": ["ast"],
"*": [
"evm.bytecode.object"
]
}
}
```

won't get the AST,

whereas one of

```

{
"*": {
"": [],
"*": [
"ast",
"evm.bytecode.object"
]
}
}
```

will.

Reproduce the issue with the two outputSelection examples and a minimal Yul input. Trace how the compiler handles file-level and contract-level AST requests in Yul versus Solidity; done means the file-level "ast" selection produces the Yul AST consistently with Solidity.

Written by the indexing model from the issue text.