wrapper: Load libraries globally instead of relative to $PWD

There's no real reason to force `./` in default paths when the library
is more and more likely to be installed on the system via `PATH` (on
Windows it's in the Vulkan SDK, on Linux `directx-shader-compiler` is
a common package).

Furthermore, stop appending this path to the optional `lib_path`
argument, if that `lib_path` doesn't **exist** as a file.  In the case
that a user specifies a (relative) path that doesn't exist, this throws
an obscure error that `libdxcompiler.so/./libdxcompiler.so` doesn't
exist because we appended a precooked filename a second time.

src/wrapper.rs

@@ -8,10 +8,10 @@ use crate::ffi::*;
 use crate::os::{HRESULT, LPCWSTR, LPWSTR, WCHAR};
 use crate::utils::{from_wide, to_wide, HassleError, Result};
 use com::{class, interfaces::IUnknown, production::Class, production::ClassAllocation, Interface};
-use libloading::{Library, Symbol};
+use libloading::{library_filename, Library, Symbol};
 use std::cell::RefCell;
 use std::ops::Deref;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 use std::pin::Pin;
 
 pub struct DxcBlob {
@@ -491,34 +491,12 @@ pub struct Dxc {
     dxc_lib: Library,
 }
 
-#[cfg(target_os = "windows")]
-fn dxcompiler_lib_name() -> &'static Path {
-    Path::new("dxcompiler.dll")
-}
-
-#[cfg(any(target_os = "linux", target_os = "android"))]
-fn dxcompiler_lib_name() -> &'static Path {
-    Path::new("./libdxcompiler.so")
-}
-
-#[cfg(target_os = "macos")]
-fn dxcompiler_lib_name() -> &'static Path {
-    Path::new("./libdxcompiler.dylib")
-}
-
 impl Dxc {
-    /// `dxc_path` can point to a library directly or the directory containing the library,
-    /// in which case the appended filename depends on the platform.
+    /// `lib_path` is an optional path to the library.  Otherwise
+    /// [`libloading::library_filename("dxcompiler")`] is used.
     pub fn new(lib_path: Option<PathBuf>) -> Result<Self> {
-        let lib_path = if let Some(lib_path) = lib_path {
-            if lib_path.is_file() {
-                lib_path
-            } else {
-                lib_path.join(dxcompiler_lib_name())
-            }
-        } else {
-            dxcompiler_lib_name().to_owned()
-        };
+        let lib_path = lib_path.unwrap_or_else(|| PathBuf::from(library_filename("dxcompiler")));
+
         let dxc_lib =
             unsafe { Library::new(&lib_path) }.map_err(|e| HassleError::LoadLibraryError {
                 filename: lib_path,
@@ -678,19 +656,11 @@ impl Dxil {
         ))
     }
 
-    /// `dxil_path` can point to a library directly or the directory containing the library,
-    /// in which case `dxil.dll` is appended.
+    /// `lib_path` is an optional path to the library.  Otherwise
+    /// [`libloading::library_filename("dxil")`] is used.
     #[cfg(windows)]
     pub fn new(lib_path: Option<PathBuf>) -> Result<Self> {
-        let lib_path = if let Some(lib_path) = lib_path {
-            if lib_path.is_file() {
-                lib_path
-            } else {
-                lib_path.join("dxil.dll")
-            }
-        } else {
-            PathBuf::from("dxil.dll")
-        };
+        let lib_path = lib_path.unwrap_or_else(|| PathBuf::from(library_filename("dxil")));
 
         let dxil_lib =
             unsafe { Library::new(&lib_path) }.map_err(|e| HassleError::LoadLibraryError {