Refactor relocation loading to allow loading relocations from external sources

### Description

Currently relocations for elf and pe backends are only read from their own relocations directory. This design is not extensible and does not allow for more relocations to be added by external sources without changing the source of cle itself.

### Alternatives

_No response_

### Additional context

Allowing other programs to add relocations allows programs to develop custom relocations that might not be suitable for general use, or might prefer not to have to use a modified version of cle.