Overhaul relocation logic#27
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Relocation: Change strategy
Previously we were searching a set of prefixes for DLLs during relocation. If we found a dll that matched the dll we were looking for (based on file name) we performed relocation based on that path. This is both dangerous and extraneous.
This is dangerous as mutli config layouts may have the same binary with the same name in mutliple different paths for different configs or variations. Since we were previously only checking the filename, this could lead to a false positive detection and bad relocation not detected until runtime.
This is extraneous as we should never need to search. We have the dll locations before and after relocation, whether from the stage to install prefix or from buildcache to buildcache, so rather than a filesystem search, we can have a linear time operation where we search through a list of relocation old->new prefix mappings.
Spack core will set an environment variable of the structure:
old_prefix|new_prefix and the compiler wrapper now composes a map out of that list and then PE files looking to relocate their internal dll references get a constant time lookup.
Based on #26