[lsp] Go to definition

This adds the "textDocument/definition" capability to the language
server. This capability only works for library names and top-level
declarations, i.e. not members or methods.

Test: go test ./...
Change-Id: I6de0ae3730ac54db7975b37f7136514bfa2255d0
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidl-misc/+/395895
Reviewed-by: Pascal Perez <pascallouis@google.com>
Reviewed-by: Benjamin Prosnitz <bprosnitz@google.com>