[BUG] npm7 does not create transitive node_module folders for dependencies referenced via file:/

I've searched release notes, however I was unable to see if this was an intended breaking change.

Current Behavior:

When referencing a dependency via file:../path/to/dep, npm@7.0.9 does not install or create the transitive node_modules folder for dep

Expected Behavior:

npm@6.12.0 will create and install the node_modules folder for the referenced dependency

Steps To Reproduce:

mkdir a b

// create the package.json files with dependencies
cat << EOF > b/package.json
{
  "name": "b",
  "version": "1.0.0",
  "dependencies": {
    "lodash": "^4.17.20"
  }
}
EOF

cat << EOF > a/package.json
{
  "name": "a",
  "version": "1.0.0",
  "dependencies": {
    "b": "file:../b"
  }
}
EOF

// create a lock file for b
cd b
npm install --package-lock-only

// install for a, creating lock file and node_modules
cd ../a
npm install

a/node_modules will contain the symlink to ../../b, but with no node_modules for b

Environment:

npm@7:

• OS: MacOS 10.14.4
• Node: v15.0.1
• npm: v7.0.9

npm@6:

• OS: MacOS 10.14.4
• Node: v12.13.0
• npm: v6.12.0

Start by running the provided reproduction with npm@7.0.9 and compare the resulting a/node_modules layout with npm@6.12.0. Confirm whether the file:../b symlink lacks b's transitive node_modules directory, then trace npm CLI's handling of file dependencies and add a regression test covering the expected installation behavior.