Render external symbols snapshot outputs

[jupblb]

Produce external_symbols.txt from index.external_symbols:

scip/scip.proto

Lines 28 to 33 in 739faa9

	// (optional) Symbols that are referenced from this index but are defined in
	// an external package (a separate `Index` message). Leave this field empty
	// if you assume the external package will get indexed separately. If the
	// external package won't get indexed for some reason then you can use this
	// field to provide hover documentation for those external symbols.
	repeated SymbolInformation external_symbols = 3;

Example output (sourcegraph/scip-go#198):

github.com/example/dep 0.1.test `github.com/example/dep`/T#
  relationship 0.1.test `sg/pr198`/Foo# implementation