Based on file content. Detects elf magic header or shebang line.

Executable bit is ignored in hash calculation, as it was before this. So
packages hashes are not changed.

Reference:
https://github.com/ziglang/zig/issues/17463#issuecomment-1984798880

Fixes: 17463

Test is here:
https://github.com/ianic/zig-fetch-test/blob/7c4600d7bb263f9b72fe3d0b70071f42be89e25c/src/main.zig#L307
(if #19500 got accepted I'll move this test to the Fetch.zig)