Add `--extern foo=path` command line flag

[brson]

cc #14468

Add a command line flag to let tools like cargo specify exactly which library to use for an extern crate statement.

It looks like --extern foo=path, where "foo" is the name of a crate, and path is the path of a rust library, without the extension (so it will look for both .rlib and .dylib).

[tomjakubowski]

Does it make sense for the rustdoc command to take this flag as well?

[alexcrichton]

Yes, in general rustdoc should mirror rustc wherever necessary. Eventually cargo will require this flag for rustdoc anyway.