introduce the 'zig fetch' command + symlink support

zig fetch [options] <url>
zig fetch [options] <path>

Fetches a package which is found at <url> or <path> into the global
cache directory, printing the package hash to stdout.

Closes #16972
Related to #14280

Additionally, this commit:

* Adds uncompressed .tar support to package fetching
* Introduces symlink support to package fetching

Author: andrewrk

lib/std/tar.zig

@@ -210,7 +210,7 @@ pub fn pipeToFileSystem(dir: std.fs.Dir, reader: anytype, options: Options) !voi
                 while (true) {
                     const temp = try buffer.readChunk(reader, @intCast(rounded_file_size + 512 - file_off));
                     if (temp.len == 0) return error.UnexpectedEndOfStream;
-                    const slice = temp[0..@as(usize, @intCast(@min(file_size - file_off, temp.len)))];
+                    const slice = temp[0..@intCast(@min(file_size - file_off, temp.len))];
                     try file.writeAll(slice);
 
                     file_off += slice.len;