[Feature] Package loader for amber

we would obviously need a package manager at some point.

i'd like to propose a system for loading packages on compiler level

with these package directories (i will refer to this as `$PKGDIR` from now on):

1. `./amber_modules`
2. `~/.local/amber_modules`
3. `/usr/lib/amber_modules`
4. if `AMBER_PATH` is set, special logic applies

when importing a path that starts with a letter, compiler should import it from `$PKGDIR/package_name/main.ab`, with this logic (pseudocode):

```ts
// pseudo code
const package_directories = [
"./amber_modules",
"~/.local/amber_modules",
"/usr/bin/amber_modules"
];

if (AMBER_PATH.is_set()) {
const paths = AMBER_PATH.split(':');
for (path of paths) {
path = path.join('amber_modules');
if (path.is_dir()) {
# append from the top
package_directories.push_front(path)
}
}
}

function resolve_import(path: string): string? {
if !path.startsWithLetter() {
return resolve_local_import(path)
}
if path.split("/").length == 1 {
path += "/main.ab";
}
if !path.endsWith(".ab") {
path += ".ab";
}

for (pkgdir of package_directories) {
if file_exists(path.join(pkgdir)) {
return path.join(pkgdir);
}
if exists(path.join(pkgdir) + "/main.ab") {
return path.join(pkgdir) + "/main.ab";
}
}

return null;
}

resolve_import("package_name"); // import "package_name" -> "$PKGDIR/main.ab"
resolve_import("package_name/subpkg"); // import "package_name" -> "$PKGDIR/subpkg.ab" | "$PKGDIR/subpkg/main.ab"
resolve_import("package_name/subpkg.ab"); // import "package_name" -> "$PKGDIR/subpkg.ab" | "$PKGDIR/subpkg/main.ab"
```

for importing files in the working directory, users should use `import "./local_file"`

implementing this will allow for creating a package manager for amber, that would be maintained independently from compiler

this will also mean treating stdlib as a package, rather than as a hardcoded language feature

### AMBER_PATH logic
AMBER_PATH is an environment variable that behaves like `PATH`: it contains paths separated by `:`. for each path, amber must check if `path/amber_modules` exists and if it does, add it to potential package directories with high priority.