Remove the "ignore_for_watch" mechanism

[ghost]

This was an old mechanism to avoid restarting the build every time Dune promotes a file. It is no longer necessary now that we have Fs_cache.

[snowleopard]

Awesome! I assume you tested that it actually works as expected :)

[ghost]

I should have :) It seems to work as expected, expect that the status line still changes to "Source file changed". Fixing that.

[ghost]

After more investigation: we can't remove this hack quite yet. The code that promotes the file tracks the source file via Fs_memo.path_digest, which has two effects:

• we don't actually do the copy if the source file is the same as the generated one
• if the user modifies or deletes the source file, Dune immediately recreates it by copying the generated file

As a result, if we remove this hack then we end up restarting the build everytime a file is promoted and its contents changes. With more work, we could still remove this hack and preserve the current behaviour. But this is a bigger change and not high priority, so let's keep the status quo for now.