fix relocatability of PackageCompiler

[KristofferC]

Fixes #573

Still need to confirm this works locally.

Edit: Confirmed.

[codecov]

Codecov Report

Merging #575 (375d653) into master (4965668) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #575   +/-   ##
=======================================
  Coverage   83.50%   83.50%           
=======================================
  Files           2        2           
  Lines         491      491           
=======================================
  Hits          410      410           
  Misses         81       81           

Impacted Files	Coverage Δ
src/PackageCompiler.jl	82.30% <100.00%> (ø)

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