fix(compiler-cli): allow relative rootDir

[literalpie]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features) N/A

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

Issue Number: #36290

What is the new behavior?

If a relative rootDir is passed in, an error is no longer thrown

Does this PR introduce a breaking change?

• Yes
• No

Other information

Building off of #36291 by @Toxicable with test added and other recommended changes.

[petebacondarwin]

Great! Thanks @literalpie - I think this unit test is enough.
Can you modify the commit message? It doesn't need to mention the previous PR particularly, IMO. Perhaps:

fix(compiler-cli): resolve `rootDirs` to absolute paths

Ensure that `rootDirs` are absolute by resolving them against the
current working directory.

Fixes #36290

[literalpie]

@petebacondarwin done!

[jbedard]

Thanks for picking this up @literalpie!

[JoostK]

I once prepped an example of how this could be testing in an "integration-like" test in JoostK@c08c753, as without such a test this is easy to regress in. @literalpie could you have a look at adding a test like that?

[JoostK]

@literalpie CI is not yet happy with the second commit; it wants it to have a short body.

[literalpie]

@JoostK i think it's better now. Thanks for your help and for your patience

[AndrewKushnir]

Presubmit.

[JoostK]

@literalpie Thanks again for your contribution!!

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