This change separates a cache package out of the
golang.org/x/tools/internal/lsp/source package. The source package now
uses an interface instead a File struct, which will allow it be reused
more easily. The cache package contains the View and File structs now.
Change-Id: Ia2114e9dafc5214c8b21bceba3adae1c36b9799d
Reviewed-on: https://go-review.googlesource.com/c/152798
Reviewed-by: Ian Cottrell <iancottrell@google.com>
