Include @types/ dependencies in dependencies not devDependencies

[everett1992]

Describe the bug
Typescript packages using aws-cdk cannot find a declaration file for module 'yargs'
To Reproduce

Create a typescript package that requires aws-cdk.

mkdir repro && cd repro
echo "{}" > package.json
npm install typescript@3.5.2 aws-cdk@0.34.0
./node_modules/.bin/tsc --init
echo "import 'aws-cdk'" > index.ts
./node_modules/.bin/tsc

node_modules/aws-cdk/lib/settings.d.ts:1:24 - error TS7016: Could not find a declaration file for module 'yargs'. '/local/home/calebev/repro/node_modules/yargs/index.js' implicitly has an 'any' type.
  Try `npm install @types/yargs` if it exists or add a new declaration (.d.ts) file containing `declare module 'yargs';`

Typescript will type check libraries by default. It will fail checking aws-cdk because cdk does not include it's dependencies types in dependencies, it includes them in devDependencies.

Customers can fix this issue by

• adding @types/yargs to their dependencies.
• adding --skipLibCheck to their typescript options.

Expected behavior
There should not be any type errors in an effectively empty package. aws-cdk should include it's type dependencies in it's dependencies.

Version:

• linux
• javascript
• 0.34.0

[RomainMuller]

I think in fact the real problem is that we should not be exporting symbols that depend on a particular 3P dependency. There is no actual need for they args declarations there (we can live with a much simpler, much more restrictive type here).