[llvm] [Offload][NFC] Rename `src/` -> `libomptarget/` (PR #126573)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> The naming of those folders are literally all over the place...We have all small letters mixed with capitalized first letter...
> 
> ¯_(ツ)_/¯

I'll move `DeviceRTL` later hopefully.

https://github.com/llvm/llvm-project/pull/126573