Implicitly add std dependency if unspecified

[mitchmindtree]

While loading a Manifest with the from_file constructor, we check for dependencies declared with either std or core packages.

If neither core or std exist, we add their associated git dependencies pointing to this sway repo at the tag equal to forc's current semver version. This way, the implicit core and std deps are pinned alongside the version of forc that fetches them, guaranteeing that the manifests are compatible and that the core and std versions are compatible with eachother.

If both core and std are supplied by the user, or if only one of either core or std are provided by the user, then we don't implicitly include either. This is because in the case that only either core or std are provided, we cannot guarantee that an implicitly included core or std would be compatible with the user-specified one.

Closes #330

Sway repo examples/tests

It's best we leave the examples and test suite with the explicit path dependencies in order to ensure that they remain tested against the current state of forc rather than the latest tagged commit. This will also avoid running into CI issues during the PRs where forc's version is updated but the tag hasn't yet been pushed.

This also adds a core dependency declaration to all tests that otherwise contained an empty [dependencies] table. By overriding either the core or std dependency manually, we disable the implicit inclusion of the std dependency.

TODO

• Pass forc semver version through to Manifest::from_file constructor in order to set tag for core and std git dependencies.
• Ensure build plan validation works with implicit core/std.

[adlerjohn]

Rather an implicitly including both core and std, why not just std? Basically 100% of cases users will only include std (ref #973). The only people using core directly should be standard library developers, and that will require local paths anyways.

[mitchmindtree]

why not just std?

Good point! I was blindly following Rust in this case where both core and std are included, but I think Rust does this to support the #![no_std] use-case which isn't really relevant for Sway.

[JoshuaBatty]

utACK.

[mitchmindtree]

Just rebased onto master after the merge of #1105 cc @JoshuaBatty

[JoshuaBatty]

LGTM

[adlerjohn]

LGTM, but would like @sezna to confirm that there's nothing nuanced we missed

[sezna]

Seems reasonable. Do you want to do the thing we mentioned regarding omitting std as a Forc.toml flag as a part of this PR, or as a follow-up? If the latter, would you mind making an issue to track it?

[adlerjohn]

Filed #1150 to track removing std from default Forc manifest