Consider variable expansion during Package parsing

In the case of podspec manifests, sometimes the version is defined in a variable and we end up capturing the version variable name as the version value rather than the actual version value itself. For example, in the case of scanning `AmplifyPlugins.podspec`
```
#
# Be sure to run `pod spec lint AWSS3StoragePlugin.podspec' to ensure this is a
# valid spec and to remove all comments including this before submitting the spec.
#
# To learn more about Podspec attributes see https://guides.cocoapods.org/syntax/podspec.html
# To see working Podspecs in the CocoaPods repo see https://github.com/CocoaPods/Specs/
#

# Version definitions
$AMPLIFY_VERSION = '1.23.0'
$AMPLIFY_RELEASE_TAG = "v#{$AMPLIFY_VERSION}"

$AWS_SDK_VERSION = '2.27.0'
$OPTIMISTIC_AWS_SDK_VERSION = "~> #{$AWS_SDK_VERSION}"

Pod::Spec.new do |s|
s.name = 'AmplifyPlugins'
s.version = $AMPLIFY_VERSION
s.summary = 'Amazon Web Services Amplify for iOS.'

...
```
scancode creates a package for `AmplifyPlugins` with a version of `$AMPLIFY_VERSION`.

It would be nice to have scancode figure out what the variable is and the proper variable value as the version.