BCL API review: Separate platform specific configuration source implementations into their own packages

[divega]

Currently the Microsoft.Framework.ConfigurationModel package contains the implementations of IConfiguration and IConfigurationSourceRoot as well as a set of "built-in" IConfigurationSource implementations:

• CommandLineConfigurationSource
• IniFileConfigurationSource
• EnvironmentVariableConfigurationSource
• MemoryConfigurationSource

We believe that the implementation of IConfiguration and IConfigurtionSourceRoot will be very easy to generalize "as is" to all platforms but some of these configuration source implementations (possibly all but MemoryConfigurationSource) make assumptions about the availability of resources (e.g. file system, environment variables, etc.) that aren't applicable to all platforms.

Hence we want to move those platform specific configuration source implementations to a separate package.

[Eilon]

Memory should stay in the main package.

All the other sources should each go into their own packages.

[Eilon]

Check whether the templates need to be updated.