Add yaml tag filepath that converts marked fields to absolute paths.

[gsquared94]

Fixes: #5204

This PR introduces a new yaml tag filepath for config.go which converts the marked field to an absolute path.

type Artifact struct {
	// Workspace is the directory containing the artifact's sources.
	// Defaults to `.`.
	Workspace string `yaml:"context,omitempty" yamltags:"filepath"`
        ...
}

type KubectlDeploy struct {
	// Manifests lists the Kubernetes yaml or json manifests.
	// Defaults to `["k8s/*.yaml"]`.
	Manifests []string `yaml:"manifests,omitempty" yamltags:"filepath"`
        ...
}

This is useful since the current code assumes all relative paths in the skaffold config are defined from the os.Getwd() directory. With #5160 we'll open up support for multiple skaffold configs. Doing an upfront file path massaging avoids having to maintain the info for base paths for each config individually which can be error prone.

It also fixes this assumption that the current working directory isn't the skaffold config location but the skaffold executable dir.
(comment on context.go)

// TODO(dgageot): this should be the folder containing skaffold.yaml. Should also be moved elsewhere.
cwd, err := os.Getwd()

Followup: After this is merged, we can remove WorkingDir from runcontext and many filepath.Joins strewn throughout the codebase become redundant.

[codecov]

Codecov Report

Merging #5194 (a1f6868) into master (35214eb) will decrease coverage by 0.06%.
The diff coverage is 65.26%.

@@            Coverage Diff             @@
##           master    #5194      +/-   ##
==========================================
- Coverage   71.82%   71.75%   -0.07%     
==========================================
  Files         387      388       +1     
  Lines       13928    14008      +80     
==========================================
+ Hits        10004    10052      +48     
- Misses       3190     3211      +21     
- Partials      734      745      +11     

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	58.82% <ø> (ø)
cmd/skaffold/app/cmd/runner.go	62.50% <20.00%> (-4.17%)	⬇️
pkg/skaffold/util/config.go	65.38% <55.55%> (-12.40%)	⬇️
pkg/skaffold/yamltags/paths.go	66.66% <66.66%> (ø)
pkg/skaffold/schema/versions.go	79.66% <100.00%> (-2.49%)	⬇️
pkg/skaffold/util/util.go	83.19% <100.00%> (-0.15%)	⬇️
pkg/skaffold/yamltags/tags.go	89.69% <100.00%> (+0.21%)	⬆️

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[gsquared94]

something's wrong with the k3d integration tests. All other PRs are failing as well.

[nkubala]

nit: we usually call this pkgutil i think

[nkubala]

did my yearly reading on handling multiple errors in go, and i realized something i hadn't before: errors.Wrap is actually meant to handle this. https://golang.org/doc/go1.13#error_wrapping

the idea is that for each successive error you encounter, you wrap it in the previous error: errors.Wrap(prevErr, newErr).

then later when you get that fully wrapped error, you continuously call errors.Unwrap until you get nil - do that in a while loop and you can unwind the errors, like popping them off a stack.

anyway, multiple error handling in go is stupid any way you slice it so i'm not gonna say you should do this in this PR, but just wanted to share my revelation :)

[gsquared94]

closing in favor of #5205. We retain the current behavior of CWD being the base dir.