[reorg]: Set up and populate the internal directory

Move "internal" crates into the `internal` directory. This first batch
includes most of sixtyfps_runtime but leaves the rendering backends
alone for now.

pre-commit applied some cleanups to the moved files:
 - Consistent newline at end of file policy
 - trimming trailing whitespace
 - Formatting Cargo.toml files.

internal/compiler/fileaccess.rs

@@ -0,0 +1,74 @@
+// Copyright © SixtyFPS GmbH <info@sixtyfps.io>
+// SPDX-License-Identifier: (GPL-3.0-only OR LicenseRef-SixtyFPS-commercial)
+
+use std::borrow::Cow;
+
+#[derive(Clone)]
+pub struct VirtualFile<'a> {
+    pub path: Cow<'a, str>,
+    pub builtin_contents: Option<&'static [u8]>,
+}
+
+impl<'a> VirtualFile<'a> {
+    pub fn read(&self) -> Cow<'static, [u8]> {
+        match self.builtin_contents {
+            Some(static_data) => Cow::Borrowed(static_data),
+            None => Cow::Owned(std::fs::read(self.path.as_ref()).unwrap()),
+        }
+    }
+
+    pub fn is_builtin(&self) -> bool {
+        self.builtin_contents.is_some()
+    }
+}
+
+pub fn load_file<'a>(path: &'a std::path::Path) -> Option<VirtualFile<'static>> {
+    match path.strip_prefix("builtin:/") {
+        Ok(builtin_path) => builtin_library::load_builtin_file(builtin_path),
+        Err(_) => path.exists().then(|| VirtualFile {
+            path: Cow::Owned(path.to_string_lossy().to_string()),
+            builtin_contents: None,
+        }),
+    }
+}
+
+mod builtin_library {
+    include!(env!("SIXTYFPS_WIDGETS_LIBRARY"));
+
+    pub type BuiltinDirectory<'a> = [&'a BuiltinFile<'a>];
+
+    pub struct BuiltinFile<'a> {
+        pub path: &'a str,
+        pub contents: &'static [u8],
+    }
+
+    use super::VirtualFile;
+
+    pub(crate) fn load_builtin_file(
+        builtin_path: &std::path::Path,
+    ) -> Option<VirtualFile<'static>> {
+        let mut components = vec![];
+        for part in builtin_path.iter() {
+            if part == ".." {
+                components.pop();
+            } else if part != "." {
+                components.push(part);
+            }
+        }
+        if let &[folder, file] = components.as_slice() {
+            let library = widget_library().iter().find(|x| x.0 == folder)?.1;
+            library.iter().find_map(|builtin_file| {
+                if builtin_file.path == file {
+                    Some(VirtualFile {
+                        path: builtin_file.path.into(),
+                        builtin_contents: Some(builtin_file.contents),
+                    })
+                } else {
+                    None
+                }
+            })
+        } else {
+            None
+        }
+    }
+}