internal/lsp: fix tiny bug in getting files by basename
internal/lsp: fix tiny bug in getting files by basename

This change fixes gopls on Windows.

Fixes golang/go#30967

Change-Id: Iba5eb6932a4acd6a574ad6970faf7c133ecb7d32
Reviewed-on: https://go-review.googlesource.com/c/tools/+/169703
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools