[clangd][modules] Complete module names after `import`

Suggesting possible module names after typing, is not implemented.

```cpp

import f // <- should suggest importable modules

int main()
{}

```

Today with `--experimental-modules-support` there is no completion.

A few consideration:

- Should probably work for `export import` too.
- Should probably handle partitions.
- Header units and 3rdparty modules like `std` ?

Start by tracing clangd's completion handling for C++ import declarations with --experimental-modules-support. Reproduce the example with `import f` and determine how module names, export imports, partitions, header units, and third-party modules should be represented; done means relevant module suggestions appear after `import`.

Written by the indexing model from the issue text.