fix: Hash time information on inode cache hit

fix: Hash time information on inode cache hit

As mentioned in discussion #1086: If the inode cache is enabled,
hash_source_code_file will on an inode cache hit fail to hash time
information if there are temporal macros in the code. This is because
hash_source_code_string (called from hash_source_code_file via
hash_source_code_file_nocache) on an inode cache miss adds time
information to the hash in case one of those macros were found. However,
on an inode cache hit the return value of hash_source_code_file will be
correctly fetched from the cache, but the hash sum will only be updated
with (the hash of) the include file and not the time information.

The fix is to let the inode cache only cache the effects of hashing the
file and checking for macros, not the hashing of time information since
that's volatile.

After the fix:

- The new do_hash_file function performs file hashing and macro
  checking. The inode cache caches this step.
- hash_source_code_file returns a Digest instead of adding data to a
  Hash.
- hash_source_code_file calls do_hash_file and then potentially hashes
  time information. If there are no temporal macros the returned digest
  will be identical to the file hash, otherwise the returned digest will
  be of a hash of file content hash + time information.

This also improves hashes that are stored in the direct mode manifest:
Previously they were always the hash of the file content hash but now
they are just the file content hash in the common case when there is no
__DATE__ or __TIMESTAMP__ macro in the file.