Binary fixes - sharp dependency

[guybedford]

This fixes the issues described in #142, which turned out to be:

1. Could not locate the bindings file. 0.21.1 issue, which was due to the bindings search happening from the id looking in pkg/lib/build of the path doing the require instead of the base pkg/build. Updated to use the package base path in the createBindings static implementation shim function.

2. The shared library path missing error - Error: dlopen(/Users/jrdn/sharp-example/dist/sharp.node, 1): Library not loaded: @rpath/libvips-cpp.dylib. The problem here is that the rpath is hard-coded into the binary, so we need to ensure that where the binary is emitted and where the shared libraries are emitted ends up being the same relative path. This is done by emitting binaries based on the package path so we output build/Releases/binary.node instead of just binary.node in the output folder, and then similarly for the dynamic library emission we ensure the same folder structure.

[codecov-io]

Codecov Report

Merging #159 into master will increase coverage by 1.06%.
The diff coverage is 94%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage    72.3%   73.37%   +1.06%     
==========================================
  Files          10       11       +1     
  Lines         473      492      +19     
==========================================
+ Hits          342      361      +19     
  Misses        131      131

Impacted Files	Coverage Δ
src/loaders/node-loader.js	100% <100%> (ø)	⬆️
src/utils/dedupe-names.js	100% <100%> (ø)	⬆️
src/utils/get-package-base.js	100% <100%> (ø)
src/utils/sharedlib-emit.js	75% <81.81%> (ø)	⬆️
src/loaders/relocate-loader.js	93.15% <93.33%> (+0.18%)	⬆️

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