[BUG] npm install -g includes devDependencies in v7

[mikeharder]

Current Behavior:

npm install -g includes devDependencies as part of the global install. This appears to be a breaking change between v6 and v7. Might be related to #2968.

Expected Behavior:

npm install -g does not include devDependencies.

Steps To Reproduce:

1. Create a directory containing a package.json with both a dependency and a devDependency:

{
  "name": "test",
  "version": "1.0.0",
  "dependencies": {
    "typescript": "^4.2.3"
  },
  "devDependencies": {
    "rimraf": "^3.0.2"
  }
}

2. Run npm install --prod to install prod dependencies. Required in npm v7 due to [BUG] npm install -g doesn't install local dependencies in v7 #2968.

3. Run npm install -g.

npm v6

# npm install -g
+ test@1.0.0
added 1 package in 0.073s

# npm ls -g --depth 1
`-- test@1.0.0 -> /app
  `-- typescript@4.2.3

npm v7

# npm install -g
added 1 package, and audited 3 packages in 403ms
found 0 vulnerabilities

# npm ls -g --depth 1
`-- test@1.0.0 -> /app
  +-- UNMET DEPENDENCY rimraf@^3.0.2
  `-- typescript@4.2.3

Environment:

• OS: Ubuntu 18.04
• Node: 15.12.0
• npm: 7.7.5

[nlf]

there's no bug here, but i do think the npm ls -g output is maybe a little confusing. unmet dependency is telling you that the package wasn't installed, but that we're aware it's part of a complete package tree. we should potentially be more careful to omit those when doing a global package list since it's intentional for them to not be present.