New command: pnpm fetch

[thynson]

This PR attempts to implement the idea discussed in #3207, by adding a new option --only-import-to-virtual-store (No idea for better option name yet)to install command.

With the feature implemented in this PR. Docker file can be simplified to

FROM node:lts
RUN npm install -g pnpm
COPY pnpm-lock.yaml ./ 
# This step will install all required package to the virtual store, without linking them to node_modules
RUN pnpm install --only-import-to-virtual-store
COPY . ./
RUN pnpm recursive install # Now this step only links packages

As long as the lockfile were not changed, the layer of RUN pnpm install --only-import-to-virtual-store can be cached.

@zkochan Would you please review and give some advice?

[thynson]

Maybe I should rename the option from --only-import-to-virtual-store to --without-package-json or --ignore-package-json, to make its use case more clear.

[zkochan]

I think --ignore-package-manifest would be good.

[zkochan]

Don't forget to run pnpm -w changeset and describe your changes.

[thynson]

I found that if I leave projects as is, the headless function has to be changed to skip symlink, I'm afraid the change would be too big.

[zkochan]

After thinking about it more, --ignore-package-manifest seems not correct because it ignores even the project manifests that are in the lockfile, to skip direct symlinks to node_modules. Maybe something like --prepare-dependencies would be better

[thynson]

I even thought it might be another low-level command that is managing the virtual store and lockfile, rather than an option of install which is just convenient for implementation.

[zkochan]

Like pnpm prepare-dependencies? I don't know

cc @pnpm/collaborators

[thynson]

pnpm preload？

[zkochan]

We use fetch in the repository and there are settings named fetch, so probably
pnpm fetch
pnpm prefetch
pnpm fetch-dependencies

[thynson]

I like pnpm prefetch，which hints it's an optmization in certain situation.

[zkochan]

I've created a poll to gather some feedback https://twitter.com/pnpmjs/status/1377343518579044372

[zkochan]

It looks like pnpm fetch is winning the poll.