Add logic to utilize MTL file in landsat subpackage

[lossyrob]

#23 adds a command to migrate and fixup the collection 2 STAC for Landsat 8 (thank you Alex!)

The usage of MTL and other sidecar files is still unimplemented. There's a lot of great discussion here about the challenges and potential for MTL conversion.

This issue is to determine the need and potential for using the MTL and other files to derive STAC data for landsat; if we should, implement it, and if it's not a good choice, remove the MTL codepath in the subpackage.

[gadomski]

Migrated to stactools-packages/landsat#1.