Add a PR check for viability and formatting

[cottsay]

This change adds a simple-and-sweet import/export round-trip to verify that all of the repository information is viable and that the format matches what vcstool exports.

This change adds some YAML format validation (including alphabetical ordering), as well as a validation pass using vcstool.

Requires #1134

[cottsay]

Oh, lame. Looks like a tag gets exported as a SHA, so Fast-CDR makes this not work right.

I'll convert this back to draft - we might be able to get the behavior we want from vcstool with a modifier.

[clalancette]

I like it.

[nuclearsandwich]

Very cool! Is there a way to run this against the current $ROSDISTRO-{,release} branches so that we don't immediately introduce errors from day 0?

[cottsay]

Is there a way to run this against the current $ROSDISTRO-{,release} branches so that we don't immediately introduce errors from day 0?

The workflow will need to be introduced into each branch that we want to protect, so we won't get any breakage on day 0.

[nuclearsandwich]

The workflow will need to be introduced into each branch that we want to protect, so we won't get any breakage on day 0

Works for me. I thought that workflows from the default branch would be a applied everywhere given the on: pull_requests: branches: '*' but I don't find any current docs that say as much.