Add basename and dirname functions

[joshuaspence]

Adds basename and dirname interpolation. I want to add a stack tag to our infrastructure, the value of which is set to ${basename(path.cwd)}. We currently use ${replace(path.cwd, "/^.+\\//", "")} instead, but this is extremeley unreadable. The existance of a basename function would be very useful for this use case.

I don't have an immediate use case for a dirname function, but it seemed reasonable to add it as well.

[ewbankkit]

Use of the path package or filepath package? It seems like filepath is correct here.

[apparentlymart]

Agreed with @ewbankkit that it probably makes sense to use filepath here, for correct portable behavior for Terraform on all platforms -- particularly on Windows.

Fortunately the two are API compatible to the extent that matters here. Would you mind switching that, @joshuaspence, or do you think there's a good reason to use the generic path package?

[joshuaspence]

Yep done. I'm not really a golang programmer, so I didn't know about filepath.

[apparentlymart]

Looks good to me! I will come back and merge this once we get that kubernetes test failure sorted out and thus we can see your change here pass green (which I'm sure it will...)

[ghost]

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