edit package files in place

Currently, the package files (and changelogs) are parsed into a representation such as an AST, mutated and converted back. This relies on the library we are using to round trip non-destructively, that is, do this process without changing parts of the file that we didn't modify.

We want to look at possibly editing the string directly, and kind of shimming in our change instead. We can perhaps still use this libraries to help, but not depend on them doing a round trip. This should address many of the issues we are seeing and major versions of dependencies changing expectations.

Start by tracing how package files and changelogs are parsed, mutated, and converted back. Compare that flow with the proposed direct string-editing approach; done should mean changes are inserted without depending on non-destructive round trips or specific dependency-version behavior.

Written by the indexing model from the issue text.