fix(read): change lstat to stat to correctly evaluate file size

[yorickvP]

I'm doing something interesting with cacache, which could be made a lot more efficient if I could symlink files into the cache. This almost works.
However, the content reader uses a couple of lstat calls during integrity checking, which explicitly read details about the symlink and not it's target, returning the wrong sizes.
Changing these calls to stat calls allows my plans to succeed, and shouldn't cause any harm to people not putting symlinks in their cacache.

[wraithgar]

lstat is definitely not what cacache should be using if its intent is to evaluate the size of the file.

Tests are going to have to be fixed.

[yorickvP]

Updated the commit messages and fixed the tests.

[wraithgar]

This looks good. Thanks for pointing this out. It was a bug.

Additionally, we were just in the middle of doing some optimizations to pacote to make it stop reading entire things into memory if they're small enough. Your PR brought this case of cacache doing the same to our attention. We can merge this, and we'll now go look into making cacache not try to read 64mb chunks into memory in one go, and stop streaming in 64mb chunks.