vlib: add a new dl.loader module, to simplify dynamic library loading, when the DLLs may be in multiple customisable locations (#17161)

examples/dynamic_library_loader/use.v

@@ -0,0 +1,35 @@
+module main
+
+// Note: This program, requires that the shared library was already compiled.
+// To do so, run `v -d no_backtrace -o library -shared modules/library/library.v`
+// before running this program.
+import os
+import dl
+import dl.loader
+
+type FNAdder = fn (int, int) int
+
+const (
+	cfolder       = os.dir(@FILE)
+	default_paths = [
+		os.join_path(cfolder, 'library${dl.dl_ext}'),
+		os.join_path(cfolder, 'location1/library${dl.dl_ext}'),
+		os.join_path(cfolder, 'location2/library${dl.dl_ext}'),
+		os.join_path(cfolder, 'modules/library/library${dl.dl_ext}'),
+	]
+)
+
+fn main() {
+	mut dl_loader := loader.get_or_create_dynamic_lib_loader(
+		key: cfolder + '/library'
+		paths: default_paths
+	)!
+	defer {
+		dl_loader.unregister()
+	}
+	sym := dl_loader.get_sym('add_1')!
+	f := FNAdder(sym)
+	eprintln('f: ${ptr_str(f)}')
+	res := f(1, 2)
+	eprintln('res: ${res}')
+}