x/tools/gopls: allow renaming arbitrary package paths

[findleyr]

This issue proposes to extend gopls' package renaming feature to prompt with the full package path of the the package (rather than just the package name), and allow arbitrary edits of the package path, potentially even across module boundaries.

(aside: this feature was originally part of #56184, but is significant enough in scope to warrant its own issue).

Specifically, we should:

• Prompt with the full package path in preparerename requests.
• Validate whether an arbitrary package path move can be allowed.
• Compute the (potentially complicated) set of workspace edits to implement the package move.

The pieces of this exists in the current renaming code, but must be generalized.