[BUG] Installing git dependencies that share dependencies with the parent package fails

[aredridel]

ehavior:

In our main repo, grommet-styleguide is a git dependency. It depends on systemjs, as does the parent package.

With npm 7, that dependency is not where it can be found by the prepare script in grommet-styleguide.

npm ERR! code 1
npm ERR! git dep preparation failed
npm ERR! command /usr/local/Cellar/node/15.0.1/bin/node /usr/local/lib/node_modules/npm/bin/npm-cli.js install --only=dev --prod --ignore-prepublish --no-progress --no-save --cache=/Users/aria/.npm/_cacache --prefer-offline=false --prefer-online=false --offline=false --before=
npm ERR! > [email protected] prepare
npm ERR! > cp -a node_modules/systemjs/dist/ public/vendor/systemjs && npm run postcss && npm run rollup
npm ERR! npm WARN invalid config before="" set in command line options
npm ERR! npm WARN invalid config Must be one of: null, valid Date string
npm ERR! cp: node_modules/systemjs/dist/: No such file or directory
npm ERR! npm ERR! code 1
npm ERR! npm ERR! path /Users/aria/.npm/_cacache/tmp/git-clone-2db8d5b0
npm ERR! npm ERR! command failed
npm ERR! npm ERR! command sh -c cp -a node_modules/systemjs/dist/ public/vendor/systemjs && npm run postcss && npm run rollup
npm ERR! 
npm ERR! npm ERR! A complete log of this run can be found in:
npm ERR! npm ERR!     /Users/aria/.npm/_cacache/_logs/2020-11-02T15_42_36_044Z-debug.log

Expected Behavior:

When installing with npm 6, this works, and grommet-styleguide has access to systemjs when the prepare script is run, and the build completes without error.

Steps To Reproduce:

git init npm-7-bug-1
git init npm-7-bug-2
cd npm-7-bug-1
npm init --yes
npm install systemjs
jq .scripts.prepare='"cat node_modules/systemjs/dist/s.js"'  package.json > new
mv new package.json 
git add package.json
git commit -m 'test'
rm -r node_modules
cd ../npm-7-bug-2
npm init --yes
npm install systemjs
npm i ../npm-7-bug-1

Environment:

• OS: MacOS 15
• Node: 15.0.1
• npm: 7.0.3