Feature request: Check other PRs Project modifications before opening a PR

It is often the case that a PR is already opened to update some compat entry, but it is blocked for some reason or needs more work. It would avoid lots of confusion if CompatHelper would check the open PRs for modifications to the `Project.toml`.

For example, if there is already a PR opened by someone setting `HTTP` from `0.9` to `1`, then CompatHelper shouldn't suggest the same modification

Start by tracing where CompatHelper decides which Project.toml modifications to suggest, then inspect how open pull requests can be checked before creating a suggestion. Use the example of an existing HTTP 0.9-to-1 change as the acceptance case: an equivalent modification already present in an open PR should not be suggested again.

Written by the indexing model from the issue text.