qt5.compile_translations contains path separator in its target names

Using `compile_translations` in a subproject leads to
```
WARNING: Target "qt5-compile-subprojects/wscommonqt/wscommonqt_en.ts" has a path separator in its name.
|This is not supported, it can cause unexpected failures and will become
|a hard error in the future.
```

Exporting an array of files from the subproject and then using it in the super project with `compile_translations` doesn't help either since the target name seems to be generated using the file name. I suppose there should be a call to `.underscorify()` somewhere in the target name creation of the `qt5` module?

Start with the Meson qt5 module's compile_translations entry point and reproduce the warning with a subproject, as described in the issue. Trace how target names are generated from translated file paths, then verify that the resulting targets no longer contain path separators and that the subproject case works without the warning.

Written by the indexing model from the issue text.