Move all dataset issues to their respective repositories

[gadomski]

Now that dataset packages have moved to their own repositories in the stactools-packages organization, we need to move dataset-specific issues to their respective repositories.

[gadomski]

Done:

• Add logic to utilize MTL file in landsat subpackage stactools-packages/landsat#1
• Include reflectance coefficients and license information stactools-packages/planet#2
• Better description of UDM in Planet conversion / output. stactools-packages/planet#3
• Add more file extension fields to Planet output stactools-packages/planet#4