Look up system dependencies (moc, replica, ic-starter) at runtime

[nmattia]

Instead of bundling dependencies in dfx itself, those dependencies could be searched for and downloaded only when necessary. This would solve the frustration expressed by @paulyoung on the forum by not needing to download dependencies during the build.

In general there are drawbacks to embedding dependencies. It is not considered good practice to download artifacts during the build, and in this case bloats dfx a fair bit. Instead, much like e.g. cargo does, dependencies could simply be checked at runtime. In the case of dfx, we could even offer to download and install those dependencies (which cargo doesn't do, i.e. it does not download rustc).

This would also make dfx a fair bit lighter, because optional dependencies (like moc) may never have to be downloaded (e.g. if the developer uses rust but doesn't use motoko).

[paulyoung]

From what I remember, the original motivation for this was to address concerns with the first-time developer experience.

We didn’t want new developers thinking they had everything they needed by having already downloaded the SDK and then later finding that they needed an internet connection when they didn’t have one.

I’m not sure if that’s a strong enough argument but I thought the context might be useful.