Switch human logging to stderr to enable pipeable validation output

[colindean]

Work toward go-vela/community#313

[colindean]

N.b. I did this with github.dev so no idea if the tests work… it was a copy-paste job for the stdout -> stderr impl

[codecov]

Codecov Report

Merging #416 (0555f52) into main (7000344) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
+ Coverage   86.00%   86.04%   +0.03%     
==========================================
  Files         132      133       +1     
  Lines        5754     5768      +14     
==========================================
+ Hits         4949     4963      +14     
  Misses        660      660              
  Partials      145      145              

Impacted Files	Coverage Δ
action/pipeline/validate.go	88.72% <100.00%> (ø)
internal/output/stderr.go	100.00% <100.00%> (ø)

[colindean]

I'm trying to write a pre-commit hook and stumbled upon this muddying the YAML output from vela validate pipeline.

[ecrupper]

@colindean So is the point of this PR to have the entire stdout be the YAML output of the pipeline and send the is valid statement to stderr? If so, I think I'd almost prefer removing that is valid statement in favor of just the compiled pipeline instead of sending them to different places.

[colindean]

If I had it my way:

• human-readable output always on stderr
• machine-readable output always on stdout
• machine-readable output emitted only when opted-in, e.g. --output-validated [filename or -, defaults to stdout]

This PR accomplishes the first two.

[ecrupper]

A couple typos. Tested this locally:

vela validate pipeline | yq -j

Error: bad file '-': yaml: line 2: mapping values are not allowed in this context

vs.

~/.../cli/release/darwin/amd64/vela validate pipeline | yq -j 

{
  "version": "1",
  "metadata": {
    "clone": false,
    "environment": [
      "steps",
      "services",
      "secrets"
    ]
  },
.... etc

Nice