CMP0115 requires that source files listed in CMake must include their
file extension, but there are cases when projects have different source
files with the same name, but one with an extension and one without.
In the current state, CMake will ignore the file without the extension
an always map it to the file with an extension.
```cmake
add_library(foo bar.c bar)
```
In the above example, the target `foo` will only recognize and depend on
`bar.c` and miss the file `bar` unless `bar` comes before `bar.c` in the
source list.
This issue also affects how custom commands emit files.
This change adds a new policy to recognize files with and without a file
extension as different files, both when building targets, and when they
are being created.
Fixes: #26058
321c647640