Relative paths do not work with `workspace.library` and `diagnostics.libraryFiles` & `.gitmodules`-related issue

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### How are you using the lua-language-server?

NeoVim

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking, Diagnostics/Syntax Checking, Libraries

### Expected Behaviour

`workspace.library` and `diagnostics.libraryFiles` work together even if `workspace.library` is a relative path on-disk. All that is needed is the path specified.

If I clone busted / luassert into my repository and point relative paths to those directories, lua-language-server should treat these directories as third-party and will ignore any issues they may have during diagnostics / type-checking.

Original context: https://github.com/jeffzi/llscheck/issues/7

### Actual Behaviour

`workspace.library` and `diagnostics.libraryFiles` seem to only omit diagnostics if it is given an absolute path. Relative paths are still read into the workspace but lua-language-server still reports issues on them anyway. Effectively `diagnostics.libraryFiles` is ignored.

### Reproduction steps

Jean-Francois nicely provided [a minimum reproduction of the issue](https://github.com/jeffzi/luals-example).

#### Reproduction Summary
The `README.md` from the above repository has concise reproduction steps but to further simplify...

- If you run `lua-language-server --check . --logpath logs`, it says that there are errors.
- There is a `.luarc.json` in the repository
- If you change the `"workspace.library": ["annotations"]` to `"workspace.library": ["$PWD/annotations"]`
- If you run `lua-language-server --check . --logpath logs` again, it succeeds

To conclude, the relative path was not respected but the $PWD-prefixed path is.

#### Experiments with .gitmodules
I noticed that if you empty out the .gitmodules file or delete it completely, **neither relative paths or absolute paths work**. `lua-language-server` will treat any issues within `busted` / `luassert` as if they are errors in my project even though they're meant to be third-party.

### Additional Notes

It would be very nice if lua-language-server removed the dependency on `.gitmodules`. A relative path to `workspace.library` and `diagnostics.libraryFiles: "Disable"` in my opinion should be all you need to provide for `lua-language-server` to know not to report diagnostics from there.

Additionally if we could allow relative paths so as to not need to add `$PWD` that would be nice.

### Log File

```json
{
"file:///home/selecaoone/repositories/luals-example/./annotations/luassert/library/luassert/mock.lua": [
{
"code": "cast-local-type",
"message": "This variable is defined as type `luassert.mock`. Cannot convert its type to `function`.\n- `function` cannot match `luassert.mock`\n- Type `function` cannot match `table`\n- Type `function` cannot match `luassert.mock`",
"range": {
"end": {
"character": 13,
"line": 15
},
"start": {
"character": 9,
"line": 15
}
},
"severity": 2,
"source": "Lua Diagnostics."
}
],
"file:///home/selecaoone/repositories/luals-example/./annotations/luassert/library/luassert/stub.lua": [
{
"code": "cast-local-type",
"message": "This variable is defined as type `luassert.stub`. Cannot convert its type to `function`.\n- `function` cannot match `luassert.stub`\n- Type `function` cannot match `table`\n- Type `function` cannot match `luassert.stub`",
"range": {
"end": {
"character": 13,
"line": 12
},
"start": {
"character": 9,
"line": 12
}
},
"severity": 2,
"source": "Lua Diagnostics."
}
]
}
```

Start with the linked minimum reproduction and its README.md, then run `lua-language-server --check . --logpath logs` using the repository's `.luarc.json`. Compare relative and `$PWD`-prefixed `workspace.library` values, including the presence or absence of `.gitmodules`; done means relative library paths and `diagnostics.libraryFiles` consistently suppress diagnostics without depending on `.gitmodules`.

Written by the indexing model from the issue text.