Read resources from multiple files

[carmal891]

What type of PR is this?

/kind feature

What this PR does / why we need it:
Read resources from multiple files

Which issue(s) this PR fixes:

Fixes #106

Does this PR introduce a user-facing change?:

The --input-file flag now supports multiple files and directories. When multiple inputs are provided, each file's conversion results are displayed under individual file headers.

[k8s-ci-robot]

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[k8s-ci-robot]

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[Stevenjin8]

My understanding here is this implementation is that the following would be more or less equivalent (i forget the specific flag names)

i2gw print --input-file a.yaml b.yaml c.yaml 

and

i2gw print --input-file a.yaml
i2gw print --input-file b.yaml
i2gw print --input-file c.yaml

I'm not super familiar with Ingress merging semantics, but could this not mean that we have duplicate resources in the output (maybe with slightly different names).

Ideally, the same set of kubernetes resources should create the same output regardless of which file each resources lives in.

[carmal891]

Thanks for highlighting this. I got your point. So duplicate resource check should happen amongst the set of yaml files when provided as a set like i2gw print --input-file a.yaml b.yaml c.yaml

[Stevenjin8]

Its more like

i2gw print --input-file a.yaml b.yaml c.yaml

and

i2gw print --input-file <(cat a.yaml b.yaml c.yaml)

should be equivalent.

[Stevenjin8]

This one is more personal, but I think we can omit this path since users can already do -input-file manifest-dir/**/*.{yaml,yml,json}, but if this is a common enough pattern in other places I'm happy to include.