use composite template's contributions path as fallback search for template inputs

[grokspawn]

Description of the change:

since using the composite template means the union of requirements from two parties, it can be confusing sometimes to understand the context of resource references.

For example, in the contribution schema olm.composite there are references to input filesystem elements in Components[].Strategy.Template.Config.Input (ref) but these may be relative or absolute references. And if relative... then relative to what?

This PR attempts to address this by fallback searching for input references relative to the cwd of the contribution file (if driven by CLI) or the Components[].Strategy.Template.Config.ContributionPath (if provided).

For example, given a contribution of the format

schema: olm.composite
components:
  - name: catalog-of-wonders
    destination:
      path: my-operator
    strategy:
      name: semver
      template:
        schema: olm.builder.semver
        config:
          input: semver-catalog-template.yaml
          output: catalog.yaml

the template expansion will look for semver-catalog-template.yaml in the CWD of the executed command. But if the file is actually in a catalog-data subdirectory but the command is executed in the parent, then it won't be solved, for e.g.

catalog-data
├── basic-catalog-template.yaml
└── contributions.yaml

The catalogs.yaml file which fulfills the olm.composite.catalogs spec may be local or remote, so it is unsuitable as a search location, but the olm.composite source is always local, which allows us to infer a possible anchor point for relative references.

In case a naiive access attempt for the specified contribution input file fails, we use the resulting anchor in a fallback attempt to resolve input references.

Motivation for the change:

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (4c69b4a) 53.79% compared to head (27ea513) 53.81%.

Files	Patch %	Lines
alpha/template/composite/builder.go	33.33%	9 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   53.79%   53.81%   +0.02%     
==========================================
  Files         108      108              
  Lines       10368    10385      +17     
==========================================
+ Hits         5577     5589      +12     
- Misses       3809     3814       +5     
  Partials      982      982              

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[everettraven]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: everettraven, grokspawn

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