feat(lockfiles): Use Cargo.lock to identify wasm-bindgen versions

[fitzgen]

fixes #270, #344

This lets us leverage cargo for semver finding and then ensure that we get the exact same version of the CLI that cargo selected. It also lets us support fuzzy dependencies like "0.2" instead of exact dependencies like "0.2.21" again.

Additionally, this lets us remove the _ vs - normalization, since cargo also handles that for us.

This is a updated version of #302. In addition to a rebase, this also removes the old Cargo.toml checking code, migrates callers to this new version, and ensures that we never check the wasm-bindgen version until after a Cargo.lock exists.

---

Make sure these boxes are checked! 📦✅

• You have the latest version of rustfmt installed and have your
  cloned directory set to nightly

$ rustup override set nightly
$ rustup component add rustfmt-preview --toolchain nightly

• You ran rustfmt on the code base before submitting
• You reference which issue is being closed in the PR text

✨✨ 😄 Thanks so much for contributing to wasm-pack! 😄 ✨✨

[fitzgen]

Crap, I forgot to add these test cases:

More cases we can handle now and should have tests for:

• running wasm-pack build in each crate within an umbrella workspace:

umbrella/
- Cargo.toml                        # just have `[workspace] members = [..]`
- foo/
- Cargo.toml
- src/
- lib.rs
- bar/
- Cargo.toml
- src/
- lib.rs

• a dependency uses wasm-bindgen, but the root crate does not:

// child/src/lib.rs
extern crate wasm_bindgen;
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn hello() -> u32 { 42 }

// parent/src/lib.rs
extern crate child;
pub use child::*;

[fitzgen]

Added the missing test cases.

[fitzgen]

Thinking about it some more, we might want to still check the crate config as the first thing we do, but have checking crate config not check wasm-bindgen versions. Then when we go to install wasm-bindgen, we look at the lockfile at tjhat time.

[ashleygwilliams]

yes i think that's a good call. crate config should def be one of the first things we do because it's a quick fix for folks who it fails for.

[fitzgen]

This is done.

[alexcrichton]

I might recommend returning Option here to leave Error for things like I/O errors and such, and then callers can determine if they care about the presence or not

[alexcrichton]

I'd recommend fs::read_to_string(...) nowadays

Ideally this would also use failure's with_context method to add the filename to the error message too

[ashleygwilliams]

closing in favor of #365. hopefully for the last time <3<3