Always use the file contents to determine asset aggregate filename hashes

Problem/Motivation

When building asset aggregate URLs, we use a combination of library versions for libraries that have them, and file contents for libraries that don't - this is to save hashing every file in an aggregate.

After #3505776: Retain asset aggregates for 45 days by default to reduce generation of identical files it is a bit more awkward if you update a file without updating a version for a library, because drush cr doesn't automatically clear the aggregate files. There are ways to disable the behaviour in local dev etc. but it can be a gotcha.

Thinking about whether to open this issue at all, I suddenly realised that all of core's own libraries are defined with VERSION which changes every patch release, even though the individual files often don't change for months or years.

This means that for core libraries, the logic is doing the opposite of what we want it to - results in more changes to aggregate filenames rather than less.

We can remove VERSION from all core libraries, but given it would also make things easier for contrib/custom themes and modules, I think always using the file contents is probably easier. Can profile it to see what the impact is.

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