Custom deps feature implemented [IN PROGRESS].

[cakine]

It's now possible to inject packages at runtime through custom .deps.json files. The files must be specified through DOTNET_CUSTOM_DEPS environment variable or through --custom-deps command line argument. It's important to notice that the command line argument overwrites the environment variable value.

For instance, if we wanted to load AppB during AppA execution:
dotnet --custom-deps ...\AppB.deps.json AppA.dll

It's possible to specify multiple files:
dotnet --custom-deps ...\AppB.deps.json;...\AppC.deps.json;...\AppD.deps.json AppA.dll

A directory path can be specified:
dotnet --custom-deps base_dir AppA.dll
Every .deps.json file contained in base_dir\shared\fx_name\fx_version will be picked.

It's also possible to mix directory and file paths:
dotnet --custom-deps base_dir1;...\AppB.deps.json;base_dir2 AppA.dll

The changes were manually tested, but it's still necessary to add unit tests to ensure that the new feature correctly follows the expected behavior.

Fixes dotnet/core-setup#1597

[dnfclas]

@cakine,
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[cakine]

@gkhanna79 @ramarag

[gkhanna79]

Closing since this is now merged.