BGA gen: Add extra text feature and straighten up output directory handling

This MR uses ten lines of code to add a feature to the BGA gen that allows us to put arbitrary text on an arbitray layer in the output footprint. The text can be anchored to one of the package's balls by ball number to make positioning easier.

I made this feature for cases such as !1076 where otherwise, someone would have to manually edit the footprint.

While adding this feature, I noticed that the way the BGA gen chooses the output library is totally weird and likely to lead to problems, and added a small fix for that as well. Previously, the BGA gen would always create two libraries called Packages_BGA and Packages_CSP, and sort all footprints it found in any yaml file into one of them based on whether it found BGA or CSP in the footprint name. Now, this could lead to a footprint from csp.yaml unintuitively ending up in Packages_BGA if its name contains the substring BGA. I think that's just weird, and changed it so that the name of the library is always the name of the yaml.

This MR is not quite ready to merge - for your convenience, it includes Uli's commits from !1076 so you can use the new OnSemi_ODCSP36_BGA-36_6.13x6.13mm_P1.0mm_Ball0.26mm fp as an example. I'll rebase and remove these commits before merging.