goose: use readable variable names

Names are inferred from types most of the time, but have a fallback for
a non-named type. Names are now also disambiguated from symbols in the
same scope.

Reviewed-by: Tuo Shan <shantuo@google.com>
Reviewed-by: Herbie Ong <herbie@google.com>

internal/goose/testdata/NamingWorstCase/foo/goose.go

@@ -0,0 +1,11 @@
+//+build gooseinject
+
+package main
+
+import (
+	stdcontext "context"
+)
+
+//goose:use provide
+
+func inject(context stdcontext.Context, err struct{}) (context, error)