Extract source resolution logic into its own source loader (#4324)

This will allow other tools to be able to reuse the typespec compiler
source resolution logic(Figure out all the imported files) without doing
a full compile and cleans up the program.ts which is doing a lot of
things.

Usage

```ts
const loader = createSourceLoader(host);

loader.importFile(resolvePath(cwd, "main.tsp", {type: "project"}));

loader.importPath("./foo.tsp", NoTarget, cwd, {type: "project"} );

loader.resolution.sourceFiles // Tsp source files loaded
loader.resolution.jsSourceFiles // Js source file loaded
```

.chronus/changes/source-loader-2024-8-3-19-23-57.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+API: Extract source resolution logic into its own source loader