Add step to install Modules from packages.config

[agc93]

Installs packages from tools/Modules/packages.config using the same logic as tools/packages.config

I've left out installing from tools/Addins/packages.config on the basis that using #addin will get Cake.exe to do that for us.

Resolves #37

[gep13]

@agc93 the only comment that I would have is that the original issue mentions installing both modules and addins. As a result, this PR doesn't fix the entire of Issue #37. I would suggest breaking that issue up into two separate issues, and update the original accordingly.

[devlead]

@agc93 question should we MD5 hash packages.config like standard bootstrapper to enable tools invalidation if cached on build agent?

[agc93]

Good point @gep13 , I'll raise a separate issue as I'm not sure we should be porting that functionality into this extension.

@devlead I feel like users should be using TF Build's built-in cache/invalidation controls for that, rather than introducing our own that might behave differently than what they would expect? Thoughts?

[devlead]

@agc93 If the update packages.config wouldn't they expect the tools/modules to be updated?

[agc93]

@devlead I suppose. I'm thinking people should be using the built-in clean behaviour of TF Build to control caching, no?

[devlead]

@agc93 go ahead as is then and we can adjust if people have issues.