Remove internal liblog

This commit deletes the internal liblog in favor of the implementation that
lives on crates.io. Similarly it's also setting a convention for adding crates
to the compiler. The main restriction right now is that we want compiler
implementation details to be unreachable from normal Rust code (e.g. requires a
feature), and by default everything in the sysroot is reachable via `extern
crate`.

The proposal here is to require that crates pulled in have these lines in their
`src/lib.rs`:

    #![cfg_attr(rustbuild, feature(staged_api, rustc_private))]
    #![cfg_attr(rustbuild, unstable(feature = "rustc_private", issue = "27812"))]

This'll mean that by default they're not using these attributes but when
compiled as part of the compiler they do a few things:

* Mark themselves as entirely unstable via the `staged_api` feature and the
  `#![unstable]` attribute.
* Allow usage of other unstable crates via `feature(rustc_private)` which is
  required if the crate relies on any other crates to compile (other than std).

src/bootstrap/bin/rustc.rs

@@ -94,6 +94,10 @@ fn main() {
             cmd.arg("-Cprefer-dynamic");
         }
 
+        if env::var_os("RUSTC_PASS_RUSTBUILD_FLAG").is_some() {
+            cmd.arg("--cfg").arg("rustbuild");
+        }
+
         // Help the libc crate compile by assisting it in finding the MUSL
         // native libraries.
         if let Some(s) = env::var_os("MUSL_ROOT") {

src/bootstrap/lib.rs

@@ -499,6 +499,19 @@ impl Build {
                  .env("RUSTC_SNAPSHOT_LIBDIR", self.rustc_libdir(compiler));
         }
 
+        // Most of the time we want to pass `--cfg rustbuild` which will flag
+        // upstream crates.io crates to compile themselves as unstable as we're
+        // going to put them in the sysroot. If we're compiling tools, however,
+        // we don't want to do that as the upstream crates.io deps may be in the
+        // crate graph and we don't want to compile them with their unstable
+        // versions.
+        match mode {
+            Mode::Tool => {}
+            _ => {
+                cargo.env("RUSTC_PASS_RUSTBUILD_FLAG", "1");
+            }
+        }
+
         // Ignore incremental modes except for stage0, since we're
         // not guaranteeing correctness acros builds if the compiler
         // is changing under your feet.`