Allow specification of packages built outside of flyem-build

[DocSavage]

Although the purpose of flyem-build is to have a consistent, version-controlled build of FlyEM apps, we can permit knowledgeable developers to point to outside packages instead of always building everything from scratch.

This can be done via a command-line option, similar to the current -D USE_PROJECT_DOWNLOAD method. Manual modules will be obtained through standard CMake find package commands.