Use blob for AppPath option

[jromero]

Now that we have blobs, it makes sense to use it for "AppPath" as a way to abstract input information to build from. The value being that it streams data from multiple source types such as tar, tgz and dir. Additionally, we could effectively add zip (jar) with little effort and it take effect
across all input types (buildpacks, lifecycle and app).

Proposed Solution

blob.New(path string, strict bool)

Goals

• Centralize input source traversing
• Enforce restrictions based on conditions such as source and platform (see below)

Additional considerations

• Incorporate support for additional compression formats (zip)
• Chmod 0777 when strict=false and path is dir on Windows

Prior Art

We have abstracted the inputs of layer information for buildpacks and the lifecycle into a new concept called blobs.

Windows Dir Restriction

• Original issue pack create-builder on Windows #181
• Additional commentary:

I feel like this type of logic belongs in the blob package. The idea was to encapsulate all logic about data formats in that package.

For now it looks like directory blobs are never allowed on windows (b/c of the perm issues). If we wanted to support directory blobs for an app dir or adding a buildpack at build time we could create strict and non-strict blob constructor options to that package.

Originally posted by @ekcasey in #279