Add 'npm explain' command

[isaacs]

Took the code used to explain ERESOLVE errors, and just made it a top-level command that will explain a given set of nodes.

This is surprisingly handy! Like, if you noticed that react is in the node_modules tree, you could do this:

$ npm explain node_modules/react
react@16.13.1 dev
node_modules/react
  react@"^16.12.0" from tap@14.10.8
  node_modules/tap
    dev tap@"^14.10.8" from the root project
  peer react@">=16.8.0" from ink@2.7.1
  node_modules/tap/node_modules/ink
    ink@"^2.6.0" from tap@14.10.8
    node_modules/tap
      dev tap@"^14.10.8" from the root project
    ink@"^2.6.0" from treport@1.0.2
    node_modules/tap/node_modules/treport
      treport@"^1.0.2" from tap@14.10.8
      node_modules/tap
        dev tap@"^14.10.8" from the root project
  peer react@"^16.0.0" from react-reconciler@0.24.0
  node_modules/tap/node_modules/react-reconciler
    react-reconciler@"^0.24.0" from ink@2.7.1
    node_modules/tap/node_modules/ink
      ink@"^2.6.0" from tap@14.10.8
      node_modules/tap
        dev tap@"^14.10.8" from the root project
      ink@"^2.6.0" from treport@1.0.2
      node_modules/tap/node_modules/treport
        treport@"^1.0.2" from tap@14.10.8
        node_modules/tap
          dev tap@"^14.10.8" from the root project
  peer react@"^16.8.6" from treport@1.0.2
  node_modules/tap/node_modules/treport
    treport@"^1.0.2" from tap@14.10.8
    node_modules/tap
      dev tap@"^14.10.8" from the root project

Or if you want to know why mkdirp is being duplicated, you could run:

$ npm explain mkdirp
mkdirp@1.0.4
node_modules/mkdirp
  mkdirp@"^1.0.4" from the root project

mkdirp@0.5.5
node_modules/node-gyp/node_modules/mkdirp
  mkdirp@"^0.5.1" from node-gyp@6.1.0
  node_modules/node-gyp
    node-gyp@"^6.1.0" from @npmcli/run-script@1.5.0
    node_modules/@npmcli/run-script
      @npmcli/run-script@"^1.5.0" from the root project
  mkdirp@"^0.5.0" from tar@4.4.13
  node_modules/node-gyp/node_modules/tar
    tar@"^4.4.12" from node-gyp@6.1.0
    node_modules/node-gyp
      node-gyp@"^6.1.0" from @npmcli/run-script@1.5.0
      node_modules/@npmcli/run-script
        @npmcli/run-script@"^1.5.0" from the root project

Definitely some overlap in use-cases with npm ls, but coming at the data in the other direction, starting with the dependency as the top of the "tree" and then showing each dependent tracking back to the root project, which makes it a better fit for the "why is this here??" questions.

cc: @ljharb I think I promised you this when we were discussing removing the _-prefixed fields from package.json files in node_modules.

[ljharb]

Could there be colors added for supporting terminals? it might help the readability of what is otherwise a large block of text

[ljharb]

also is there a way it could be a table instead, with the react@"^16.12.0" from tap@14.10.8 on one column and the node_modules/tap on another? (since the only time the actual path will matter is if there's more than one instance of that package in the tree)

[isaacs]

Could there be colors added for supporting terminals?

Yes, there are colors.

also is there a way it could be a table instead, with the react@"^16.12.0" from tap@14.10.8 on one column and the node_modules/tap on another?

The challenge there is that the indentation can get pretty deep, and so putting too much horizontally can be a problem, especially when these node explanations appear in error logs for ERESOLVE failures.

[isaacs]

Oops forgot to push the rebase to remove all those debug commits. Should have this ready to land shortly.

[isaacs]

Ok, this is now ready for review.

[isaacs]

Landed 5e49bda