package fetching: path separators in symlinks are not normalized

[andrewrk]

Here you can see that symlink data is passed directly to the hasher:

zig/src/Package/Fetch.zig

Lines 1430 to 1433 in ca690ff

	.sym_link => {
	const link_name = try dir.readLink(hashed_file.fs_path, &buf);
	hasher.update(link_name);
	},

This leads to incorrect package hashes on Windows when the unpacked directory of files has any symlinks with path separators in them.

Instead, on Windows only, back slashes need to be normalized into forward slashes.

Example - this is the expected results, and what happens on Linux:

$ zig fetch https://github.com/mitchellh/zig-build-macos-sdk/archive/166ff65bb7688c0a08c4ea98166ec8300aa34128.tar.gz
1220bfe1b32491a3448dff55c1dc0c12aa573f1e5bdb0a962bfbadaaa59e4833798b

However, on Windows, it gives an incorrect result:

PS C:\> zig.exe fetch https://github.com/mitchellh/zig-build-macos-sdk/archive/166ff65bb7688c0a08c4ea98166ec8300aa34128.tar.gz
1220505223f0f4e7208583416b19e3c913992218062b2ba8e41018364e7a5a8e93ae