upper-bound-check to work with google-cloud-java's split repository in more generic way

Regarding google-cloud-java's split repositories, as of now the upper-bound-check (https://github.com/googleapis/google-cloud-java/pull/6606/files) does not work for individual libraries, because the libraries-bom only references google-cloud-bom's version. It cannot update individual library's versions referenced by google-cloud-bom.

@chingor13 says:

> it seems like we might be able to generalize it to check for arbitrary artifacts with any bom
so like the pubsub repo could check itself in google-cloud-bom

Note that, contrary to upper-bound-check, Linkage Monitor should work for the individual repositories.

從 upper-bound-check 的實作和連結的 google-cloud-java PR 開始，接著追蹤它如何處理 libraries-bom 和 google-cloud-bom。當個別程式庫 repository 能夠根據任何相關 BOM 檢查任意 artifact，並更新透過該 BOM 參照的版本，同時 Linkage Monitor 的行為維持不變時，工作就完成了。

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