npm / arborist / #221

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fix: follow symlinks on build-ideal-tree add

Symlinks were being tracked as invalid when loading actual trees since
their realpath wouldn't match, given that build-ideal-tree was just
preserving the fetchSpec read from npm-package-arg when adding new
packages to the ideal tree.

This change fixes it by properly parsing and following symlinks
realpaths at the moment they're added to the ideal tree.

2254 of 2254 branches covered (100.0%)

Branch coverage included in aggregate %.

3307 of 3307 relevant lines covered (100.0%)

521.2 hits per line