Refactor relocation loading to allow loading relocations from external sources

[twizmwazin]

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.

[rhelmot]

I don't think this is a concern - there are a limited number of ELF relocations. Once they are all supported, there is no more extension needed.

[twizmwazin]

One other benefit to this that I neglected to mention is that by loading relocations as python objects rather than files on the filesystem, it would remove special configuration needed in pyinstaller downstream. Loading python code from the filesystem rather through python's standard import system is generally just a bad practice.

[rhelmot]

That can be solved by removing the autoimport stuff and just enumerating the imports.