Add TODO for Go import path

Author: anacrolix

TODO

@@ -4,3 +4,4 @@
  * Add hinting for value alignment (I'm sure there's some equation that determines if it's likely to be worthwhile to align a value. Possibly 2048, 4096 or 8192 would be appropriate for a 4096 block size?
  * Is greedy end possible? If a value exists above the region we're punching out, then we can extend up too. -> This is not possible without synchronizing with file cloning. Currently that requires holding a write lock on the manifest.
  * Support file cloning without holding the manifest write lock. This might be done with a "clone/read" lock file.
+ * Investigate using Go 1.25's alternate location go.mod thingy so we can have "github.com/anacrolix/possum" as the import.