Resolve all filepaths to absolute in 'skaffold diagnose'

[nkubala]

Description
this change forces absolute path resolution for all paths provided in a skaffold.yaml when running skaffold diagnose.

User facing changes (remove if N/A)
paths provided in a skaffold.yaml will now always be resolved to absolute.

given our getting-started example:

Before:

➜ skaffold diagnose --yaml-only
apiVersion: skaffold/v2beta16
kind: Config
build:
  artifacts:
  - image: skaffold-example
    context: .
    docker:
      dockerfile: Dockerfile
  tagPolicy:
    gitCommit: {}
  local:
    concurrency: 1
deploy:
  kubectl:
    manifests:
    - k8s-*
  logs:
    prefix: container

After:

➜ skaffold diagnose --yaml-only
apiVersion: skaffold/v2beta16
kind: Config
build:
  artifacts:
  - image: skaffold-example
    context: /Users/nkubala/Development/skaffold/examples/getting-started
    docker:
      dockerfile: Dockerfile
  tagPolicy:
    gitCommit: {}
  local:
    concurrency: 1
deploy:
  kubectl:
    manifests:
    - /Users/nkubala/Development/skaffold/examples/getting-started/k8s-*
  logs:
    prefix: container

the purpose of skaffold diagnose is to inspect exactly what skaffold is doing at runtime - this change makes which files skaffold is using more clear. additionally, this makes the output of skaffold diagnose --yaml-only more useful in successive skaffold runs outside of the root directory (specifically, providing it to skaffold render).

[codecov]

Codecov Report

Merging #5791 (e41b3fb) into master (d14f34d) will increase coverage by 0.00%.
The diff coverage is 40.00%.

@@           Coverage Diff            @@
##           master    #5791    +/-   ##
========================================
  Coverage   70.78%   70.79%            
========================================
  Files         432      436     +4     
  Lines       16242    16357   +115     
========================================
+ Hits        11497    11580    +83     
- Misses       3901     3923    +22     
- Partials      844      854    +10     

Impacted Files	Coverage Δ
pkg/skaffold/build/jib/init.go	83.05% <0.00%> (-4.45%)	⬇️
pkg/skaffold/build/jib/jib.go	69.46% <0.00%> (-1.08%)	⬇️
pkg/skaffold/config/options.go	100.00% <ø> (ø)
pkg/skaffold/parser/config.go	79.56% <ø> (-0.13%)	⬇️
pkg/skaffold/build/jib/jvm.go	55.55% <55.55%> (ø)
cmd/skaffold/app/cmd/diagnose.go	64.51% <100.00%> (+1.18%)	⬆️
pkg/skaffold/color/formatter.go	96.00% <0.00%> (-4.00%)	⬇️
pkg/skaffold/docker/image.go	78.34% <0.00%> (-1.39%)	⬇️
pkg/skaffold/schema/profiles.go	88.46% <0.00%> (-1.15%)	⬇️
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
... and 13 more

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