gopls/internal/span: move internal/span into gopls

Spans are logically part of gopls, but could not be moved
into the gopls module because of a number of depenencies
from packagestest, analysis, and internal/diff.
Those edges are now broken.

Change-Id: Icba5ebec6b27974f832a1186120a4b87d5f87103
Reviewed-on: https://go-review.googlesource.com/c/tools/+/440176
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/gopls/internal/lsp/cache/mod_tidy.go b/gopls/internal/lsp/cache/mod_tidy.go
index e471a9b..63346dc 100644
--- a/gopls/internal/lsp/cache/mod_tidy.go
+++ b/gopls/internal/lsp/cache/mod_tidy.go
@@ -19,11 +19,11 @@
 	"golang.org/x/tools/gopls/internal/lsp/command"
 	"golang.org/x/tools/gopls/internal/lsp/protocol"
 	"golang.org/x/tools/gopls/internal/lsp/source"
+	"golang.org/x/tools/gopls/internal/span"
 	"golang.org/x/tools/internal/event"
 	"golang.org/x/tools/internal/event/tag"
 	"golang.org/x/tools/internal/gocommand"
 	"golang.org/x/tools/internal/memoize"
-	"golang.org/x/tools/internal/span"
 )
 
 // ModTidy returns the go.mod file that would be obtained by running