[vela-hugo plugin] Add ability to source themes from go.mod

[ecrupper]

Description

Currently, the vela-hugo plugin only uses a directory in the source code to discover themes. This requires users to have their themes in their code base rather than sourced from go.mod, which is another option.

Value

Better support of hugo with the plugin.

Definition of Done

Users can source themes via the go.mod file

Effort (Optional)

Impacted Personas (Optional)

[galacticgumshoe]

The go.mod approach leverages Hugo Modules for resolving the theme, which is so much simpler (and now preferred) over the previous two methods (either cloning or using Git submodule). I was attempting to build my project in Vela with this plugin where I had recently migrated my Docsy theme from using Git submodules (where a themes dir is where those files get downloaded into) to this newer Hugo Modules approach, as outlined here. You'll notice that in the migration docs, it has you perform this step when your repo had Previously used Docsy theme as git clone

rm -rf themes/docsy

or when the repo had Previously used Docsy theme as git submodule

git rm -rf themes/docsy

Either way you end up removing all of the theme-related content from your GHE repo, and so at build time you just need to perform:

hugo mod get
hugo mod tidy

Upgrading to a new version of the theme is simpler now too, it's just:

hugo mod get -u github.com/google/docsy@v0.12.0 <--or whatever version you want to upgrade to

[galacticgumshoe]

Here's the failure when trying to use this plugin against my project's repo now that it has moved to the Hugo Module approach:

--
  | "canonical": "v0.7.1",
  | "major": 0,
  | "minor": 7,
  | "patch": 1,
  | "metadata": {
  | "architecture": "amd64",
  | "build_date": "2025-06-05T20:29:18Z",
  | "compiler": "gc",
  | "git_commit": "5ec39d9d6a25c520ea4cb5196d3865bb0df0e459",
  | "go_version": "go1.23.6",
  | "operating_system": "linux"
  | }
  | }
  | time="2025-06-09T16:46:03Z" level=info msg="Vela Hugo Plugin" code="https://github.com/go-vela/vela-hugo" docs="https://go-vela.github.io/docs/plugins/registry/pipeline/hugo" registry="https://hub.docker.com/r/target/vela-hugo"
  | time="2025-06-09T16:46:03Z" level=info msg="using extended hugo binary"
  | time="2025-06-09T16:46:03Z" level=info msg="downloading hugo version from: https://github.com/gohugoio/hugo/releases/download/v0.147.7/hugo_extended_0.147.7_linux-amd64.tar.gz?checksum=file:https://github.com/gohugoio/hugo/releases/download/v0.147.7/hugo_0.147.7_checksums.txt"
  | time="2025-06-09T16:46:08Z" level=fatal msg="no theme directory found @ themes"