import names are not unified

## Description

The `as` [import keyword](https://docs.soliditylang.org/en/latest/grammar.html#a4.SolidityParser.importDirective) used in `import "./file.sol" as name` creates names considered distinct from other imports of the same file under the same name.

They should be unified when they wrap the exact same file.

## Environment

solc 0.8.28

## Steps to Reproduce

```solidity
// imported.sol
contract C {}

// file1.sol
import "./imported.sol" as Imported;

// file2.sol
import "./imported.sol" as Imported;

// toplevel.sol
import "./file0.sol";
import "./file1.sol";

// Compilation fails with `Identifier already declared`.
```

Contrast the above with the 2 examples below, which both work:

```solidity
// imported.sol
contract C {}

// file1.sol
import {C} from "./imported.sol";

// file2.sol
import {C} from "./imported.sol";

// toplevel.sol
import "./file0.sol";
import "./file1.sol";

// Compilation succeeds.
```

and

```solidity
// imported.sol
contract C {}

// file1.sol
import {C as D} from "./imported.sol";

// file2.sol
import {C as D} from "./imported.sol";

// toplevel.sol
import "./file0.sol";
import "./file1.sol";

// Compilation succeeds.
```

Start by reproducing the failure with solc 0.8.28 using imported.sol, file1.sol, file2.sol, and toplevel.sol from the report, then trace the compiler's import and name-resolution handling. Done means repeated `import "./imported.sol" as Imported` imports are unified when they refer to the exact same file, while the two working examples continue to compile.

Written by the indexing model from the issue text.