Refactor SDK location

See NPANDAY-505 for more details on the initial proposal. In the interim, several probing paths have been added to the compile/executable plugins metadata. However, these suffer some failings:

• imprecise location of the right framework in some cases (e.g. vbc always uses the latest tools version, matches 4.0 even when targetting 2.0, etc.)
• no way to specify the tools version vs. the framework (target) version
• duplication of probing paths that need to be updated for new SDK releases

We should pick up on the suggestions in the original ticket to create an SDK description that can find the right paths for MSBuild tools and for NET FX tools based on a given tools version, framework version, SDK version (if applicable/desired) and executable. This can then be referenced by more succinct plugin definitions (one per executable).

The same API could be used for other SDKs, like Azure and Web Deploy.