Internal improvement: Make consistent use of import type in most TS packages

[haltman-at]

In our Typescript packages, we've often been using import when we could be using import type. This PR is the start of an attempt at fixing that. Basically I went over most of the TS packages and replaced every import I could find that was only importing types with import type (I also did the same thing with exports where it seemed appropriate).

I did not touch codec or decoder because #3974 is still outstanding and I don't want even more merge conflicts.

I did not touch db or db-kit because they're large and confusing and I don't know them well, and I'm hoping someone else can handle those. Edit: According to @gnidan, these packages already do this properly, so I don't need to do anything there! Hooray!

I did not touch testing code, because I did not really see a need.

While doing this, I also noticed that:

1. Outside of testing, @truffle/preserve-fs only uses @truffle/preserve for types, and
2. Outside of testing, @truffle/preserve-to-ipfs only uses cids for types

So I reclassified both of those dependencies as devDependencies. The former seems a little weird, but that's what the code says! I don't know these packages really.

[rkalis]

Wrt reclassifying e.g. @truffle/preserve as a devDependency in @truffle/preserve-fs:

@truffle/preserve is used in the exported types of @truffle/preserve-fs, e.g.:

import * as Preserve from "@truffle/preserve";
import { TargetPathOptions } from "./types";
export declare function targetPath(options: TargetPathOptions): Preserve.Process<Preserve.Target>;

If we set @truffle/preserve to a devDependency, I'm pretty sure that TypeScript will complain that it cannot find @truffle/preserve when you install @truffle/preserve-fs without also installing @truffle/preserve since it does not automatically get installed any more, but TypeScript does need it to make sense of the exported types.

[eggplantzzz]

@rkalis Is that true? All of the type info gets stripped out during compilation according to the docs. "It always gets fully erased" https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-8.html

I guess we can test this.

[rkalis]

I'm not 100% what the interaction will be in this case, but the snippet I pasted above is the compiled .d.ts output for @truffle/preserve-fs. So in a different project if you install @truffle/preserve-fs, TypeScript will look at those declaration files and not understand the Preserve.* types, since it can't find module @truffle/preserve. You're right that the compiled JS code doesn't care about any of the types, but if the consuming project uses TypeScript, I think it will fail compilation because it can't interpret the declaration file without having @truffle/preserve installed.

We can test the interaction to be sure.

[haltman-at]

I mean, if you only need the dependency to compile, then it's a devDependency, right?

[rkalis]

Yes, but I don't think the package's devDependencies get installed when installing a package, which means that type checking would fail when compiling a project that uses @truffle/preserve-fs. I might be wrong there though.

[gnidan]

Let's play it safe and keep it a regular dep for now.

[haltman-at]

OK, reverted that commit.