Fix issue 17392 - Add Dub file for the lexer and parser

The Dub file is intended to make the the compiler available as a
library through Dub.

dub.sdl

@@ -0,0 +1,50 @@
+name "dmd"
+description "The DMD compiler"
+authors "Walter Bright"
+copyright "Copyright © 1999-2017, Digital Mars"
+license "BSL-1.0"
+
+targetType "none"
+dependency ":parser" version="*"
+
+subPackage {
+  name "root"
+  targetType "library"
+  sourcePaths "src/ddmd/root"
+}
+
+subPackage {
+  name "lexer"
+  targetType "library"
+  sourcePaths
+
+  sourceFiles \
+    "src/ddmd/console.d" \
+    "src/ddmd/entity.d" \
+    "src/ddmd/errors.d" \
+    "src/ddmd/globals.d" \
+    "src/ddmd/id.d" \
+    "src/ddmd/identifier.d" \
+    "src/ddmd/lexer.d" \
+    "src/ddmd/tokens.d" \
+    "src/ddmd/utf.d"
+
+  // stringImportPaths cannot be used because it will make Dub extremely slow
+  // https://github.com/dlang/dub/issues/1199
+  dflags "-J$PACKAGE_DIR"
+
+  dependency "dmd:root" version="*"
+}
+
+subPackage {
+  name "parser"
+  targetType "library"
+  sourcePaths
+
+  sourceFiles \
+    "src/ddmd/astbase.d" \
+    "src/ddmd/astbasevisitor.d" \
+    "src/ddmd/parse.d"
+
+  dependency "dmd:lexer" version="*"
+}