Implement transitive support for NPM dependencies

This commit implements [RFC 8], which enables transitive and transparent
dependencies on NPM. The `module` attribute, when seen and not part of a
local JS snippet, triggers detection of a `package.json` next to
`Cargo.toml`. If found it will cause the `wasm-bindgen` CLI tool to load
and parse the `package.json` within each crate and then create a merged
`package.json` at the end.

[RFC 8]: https://github.com/rustwasm/rfcs/pull/8

crates/cli-support/src/lib.rs

@@ -1,7 +1,7 @@
 #![doc(html_root_url = "https://docs.rs/wasm-bindgen-cli-support/0.2")]
 
 use failure::{bail, Error, ResultExt};
-use std::collections::BTreeSet;
+use std::collections::{BTreeSet, BTreeMap};
 use std::env;
 use std::fs;
 use std::mem;
@@ -329,6 +329,8 @@ impl Bindgen {
                 start: None,
                 anyref: Default::default(),
                 snippet_offsets: Default::default(),
+                npm_dependencies: Default::default(),
+                package_json_read: Default::default(),
             };
             cx.anyref.enabled = self.anyref;
             cx.anyref.prepare(cx.module)?;
@@ -366,6 +368,16 @@ impl Bindgen {
                     .with_context(|_| format!("failed to write `{}`", path.display()))?;
             }
 
+            if cx.npm_dependencies.len() > 0 {
+                let map = cx
+                    .npm_dependencies
+                    .iter()
+                    .map(|(k, v)| (k, &v.1))
+                    .collect::<BTreeMap<_, _>>();
+                let json = serde_json::to_string_pretty(&map)?;
+                fs::write(out_dir.join("package.json"), json)?;
+            }
+
             cx.finalize(stem)?
         };
 
@@ -701,4 +713,11 @@ impl OutputMode {
             _ => false,
         }
     }
+
+    fn bundler(&self) -> bool {
+        match self {
+            OutputMode::Bundler => true,
+            _ => false,
+        }
+    }
 }