dts infinite loop

I think #836 may have introduced an infinite loop when compiling type declarations in watch mode. I get this in 6.6.3 when running my project with the `watch` and `dts` flags:

If I rollback to 6.6.2 it works as expected and only runs once.

Reproduce the report with tsup 6.6.3 using the watch and dts flags, then compare behavior with 6.6.2. Inspect the changes from issue #836 and the 13-comment discussion to locate the declaration-watch loop; done means watch mode performs one declaration build instead of looping indefinitely.

Written by the indexing model from the issue text.