Relocation of `PT_INTERP` in ELF files already happens to work from long to short path, thanks to generic binary relocation (i.e. find and replace). This PR improves it:
1. Adds logic to grow `PT_INTERP` strings through patchelf (which is only useful if the interpreter and rpath paths are the _only_ paths in the binary that need to be relocated)
2. Makes shrinking `PT_INTERP` cleaner. Before this PR when you would use Spack-built glibc as link dep, and relocate
executables using its dynamic linker, you'd end up with
```
$ file exe
exe: ELF 64-bit LSD pie executable, ..., interpreter /////////////////////////////////////////////////path/to/glibc/lib/ld-linux.so
```
With this PR you get something sensible:
```
$ file exe
exe: ELF 64-bit LSD pie executable, ..., interpreter /path/to/glibc/lib/ld-linux.so
```
When Spack cannot modify the interpreter or rpath strings in-place, it errors out without modifying the file, and leaves both tasks to patchelf instead.
Also add type hints to `elf.py`.
28eea2994f