We are tied to using a SwiftPM that matches the toolchain, so upgrade
from 0.3.0 to .branch("master") and add a pins file to manage updating
that dependency.
This was driven by changes that broke loading packages being developed
with newer version of SwiftPM when using the 0.3.0 tag of libSwiftPM.
However, the changes seem to work when going in the other direction, so
using the newer libSwiftPM hasn't caused any known regressions for using
older toolchain versions (not guarantteed and not tested extensively
though).
This fixes using the latest (November 13) toolchain snapshot on macOS.
On Linux there are other issues not specific to LSP.
