Added a View interface to the source package, which allows for reading
of other files (in the same package or in other packages). We were
already reading files in jump to definition (to handle the lack of
column information in export data), but now we can also read files in
diagnostics, which allows us to determine the end of an identifier so
that we can report ranges in diagnostic messages.
Updates golang/go#29150
Change-Id: I7958d860dea8f41f2df88a467b5e2946bba4d1c5
Reviewed-on: https://go-review.googlesource.com/c/154742
Reviewed-by: Ian Cottrell <iancottrell@google.com>
f344c7530c