Migrate build-rs to the Cargo repo

This pulls in https://github.com/cad97/build-rs at eb389d1 with the following changes:
- `Cargo.toml` metadata
- Removal of `.github`, `.gitignore`, `Cargo.lock`

We'll need to integrate `test-lib` into our processes but that seemeed
more invasive, so I wanted to leave that for a future PR.

Infra changes are being coordinated in https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Transfering.20.60build-rs.60.20crate.20to.20rust-lang/near/480779960

Context: per [Cargo's charter](https://doc.crates.io/contrib/team.html#decision-process), we approved this transfer in an [FCP](rust-lang#12432 (comment)).

Fixes rust-lang#12432

crates/build-rs/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "build-rs"
+version = "0.2.0"
+rust-version.workspace = true
+edition.workspace = true
+license.workspace = true
+homepage.workspace = true
+repository.workspace = true
+description = "API for writing Cargo `build.rs` files"
+
+[features]
+unstable = []
+
+[lints]
+workspace = true

crates/build-rs/src/allow_use.rs

@@ -0,0 +1,17 @@
+use std::sync::OnceLock;
+
+fn rust_version_minor() -> u32 {
+    static VERSION_MINOR: OnceLock<u32> = OnceLock::new();
+    *VERSION_MINOR.get_or_init(|| {
+        crate::input::cargo_pkg_rust_version()
+            .split('.')
+            .nth(1)
+            .unwrap_or("70")
+            .parse()
+            .unwrap()
+    })
+}
+
+pub(crate) fn double_colon_directives() -> bool {
+    rust_version_minor() >= 77
+}