Consolidate crates.io convention section (rust-lang#1326)

Author: iwanders

src/doc/rustc-dev-guide/src/conventions.md

@@ -101,10 +101,7 @@ if foo {
 
 # Using crates from crates.io
 
-It is allowed to use crates from crates.io, though external
-dependencies should not be added gratuitously. All such crates must
-have a suitably permissive license. There is an automatic check which
-inspects the Cargo metadata to ensure this.
+See the [crates.io dependencies][crates] section.
 
 <a name="er"></a>
 
@@ -152,3 +149,4 @@ to the compiler.
   crate-related, often the spelling is changed to `krate`.
 
 [tcx]: ./ty.md
+[crates]: ./crates-io.md

src/doc/rustc-dev-guide/src/crates-io.md

@@ -21,4 +21,4 @@ to the compiler unless there is a good reason to do so.
 The `tidy` tool has [a list of crates that are allowed]. To add a
 dependency that is not already in the compiler, you will need to add it to the list.
 
-[a list of crates that are allowed]: https://github.com/rust-lang/rust/blob/19ecce332e56941ea0dd2a805270faa102acdb14/src/tools/tidy/src/deps.rs#L59
+[a list of crates that are allowed]: https://github.com/rust-lang/rust/blob/9d1b2106e23b1abd32fce1f17267604a5102f57a/src/tools/tidy/src/deps.rs#L73