Include optionalDependencies for pnpm-lock.yaml parsing

[phantomrose96]

I'm addressing two small issues:

1. Packages may specify "optionalDependencies". Currently the implementation for "parsePnpmLock" does not account for those. I've added optionalDependencies to the LockDependency type and pipe them through in parsePnpmLock
2. In a rush repository, internal packages have their version field undefined in the pnpm-lock.yaml file. This is resulting in parsed lockfile entries reading as "foo@undefined". I think it's better form to not append "@undefined" and just use the name as the key. I've edited "nameAtVersion" to reflect this. However I see there's a larger work item to handle internal packages for rush repos. If it doesn't seem worth including this partial fix for handling .tgz files, I can remove it. Removing this one.

Validation:

• built and linked locally to a project making use of "parseLockFile". Validated behaviors described above.

[ghost]

All CLA requirements met.

[ecraig12345]

If you're still interested in making this change, can you please merge with master and generate a change file (run yarn and then yarn change)? I can't update the PR for you in this case since it's made from your fork's master branch.