Move most of the root package into a subcrate. (#1445)

This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).

docker/Dockerfile.nu-base

@@ -7,6 +7,7 @@ ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update && apt-get install -y libssl-dev \
     libxcb-composite0-dev \
     pkg-config \
+    libx11-dev \
     curl
 
 ARG RELEASE=false
@@ -18,7 +19,7 @@ COPY . /code
 RUN echo "##vso[task.prependpath]/root/.cargo/bin" && \
     rustc -Vv && \
     if $RELEASE; then cargo build --release; \
-                   cp target/release/nu /usr/local/bin; \   
+                   cp target/release/nu /usr/local/bin; \
                  else cargo build; \
                    cp target/debug/nu /usr/local/bin; fi;
 ENTRYPOINT ["nu"]