Improve debugger

[bhcleek]

reset current directory after each test

use go list to get import path via go#package#FromPath

fix uses of g:go_debug_commands

• use go#config#DebugCommands() when reading the value so that it
  doesn't have to be set before the read.
• return an empty list instead of an empty dictionary for its default
  value.

Shutdown and detach from delve correctly

Use a single connection to delve to simplify detaching properly. The
previous implementation was making a connection to delve for each
request and was never closing any of them. Instead, create a single
connection to delve and reuse it for each rpc call. This allows vim-go
to not have to pass --accept-multiclient, which in turn shutsdown
delve when detach is called.

As part of this work, call_jsonrpc was refactored to not deal with
callbacks explicitly. Because Neovim requires a callback to get data
from a socket, the on_data callback was moved into out_cb and
improved to deal with the edge cases that Neovim has with regard to
partial messages.

delay changing working directory for dlv test

Delay changing the working directory for debugging tests only if the
package to be debugged could be determined.

Return early and show an error if the package could not be determined.

add debug tests

Add debug tests to ensure that :GoDebugStart and :GoDebugStop work as
expected with no arguments, a relative package and package specified
with its full import path

The test package source file is in a package named debugmain instead of
following the usual pattern (e.g. debug), because delve seems to choke
pretty hard on when the package being debugged is named debug...