Rework module layout

The psm1s are getting big!

 We should move over to the module layout that has been pretty popular and successful:

SecretManagement.LastPass.Extension.psd1
SecretManagement.LastPass.Extension.psm1
Public/
* Get-Secret.ps1
* etc

Private/
* Invoke-LpassInternal.ps1
* etc

where the psm1 simply dot-sources all of the ps1s. Example: https://github.com/PowerShell/Polaris
