Add validation for files with a .(resource-name).yml extension

[shreyas-goenka]

Changes

We want to encourage a pattern of only specifying a single resource in a YAML file when an .(resource-type).yml (like .job.yml) is used. This convention could allow us to bijectively map a resource YAML file to it's corresponding resource in the Databricks workspace.

This PR:

1. Emits an info diagnostic when we detect this convention is being violated. We can promote this to a warning when we want to encourage this pattern more strongly.
2. Visualises the info diagnostics in the bundle validate command.

Tests

Unit tests and manually. Here's what an example output looks like:

➜  bundle-playground git:(master) ✗ cli bundle validate
Info: We recommend only defining a single job in a file with the .job.yml extension.
The following resources are defined or configured in this file:
  - bar (job)
  - foo (job)

  at resources.jobs.bar
     resources.jobs.foo
  in foo.job.yml:13:7
     foo.job.yml:5:7

[shreyas-goenka]

Triggered nightlies against this PR...

[shreyas-goenka]

Nightlies are green!