x/tools/gopls: rename on package identifer produces unhelpful error message when renaming to package name that already exists

[madelinekalil]

test/test.go
package test
test2/test2.go
package test2

Initiate a rename request on "test" package, enter "test2", and observe that some edits are applied to the file but an error pops up with "Rename failed to apply edits"
We should handle the case of conflicting directory names while implementing package move. A rename like this could be interpreted as an attempt to move the code in package test into package test2 (package merging), or we could just throw an error saying it's unsupported.

[gopherbot]

Change https://go.dev/cl/712160 mentions this issue: gopls/internal/golang: implement package move