`dpm test --files` should resolve paths relative to the package root

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What is the problem you want to solve?

When I set the package with --package-root or DAML_PACKAGE, dpm test --files still resolves the file path relative to the current directory. Thus, I must write the package root two times.

This works:

DAML_PACKAGE=test/some-package-test/ dpm test --files test/some-package-test/daml/Path/To/File/Test.daml

This fails:

$ DAML_PACKAGE=test/some-package-test/ dpm test --files daml/Path/To/File/Test.daml
...
File:     /home/user/repos/project/daml/Path/To/File/Test.daml
Hidden:   no
Range:    1:1-2:1
Source:   compiler
Severity: DsError
Message: 
/home/user/repos/project/daml/Path/To/File/Test.daml: openBinaryFile: does not exist (No such file or directory)
damlc: Error when running Shake build system:
BadDependency "GeneratePackage"

What is the solution you would propose?

If --package-root or DAML_PACKAGE sets the package, resolve a relative path in --files relative to the package root.

If the file is not in the package, show a clear error that gives the package root and the resolved path. The current error (BadDependency "GeneratePackage") does not tell the user what is wrong.

Describe alternatives you've considered

1. Write the full path from the current directory. This works, but the path repeats the package root.
2. cd into the package and use a path relative to it. This makes --package-root and DAML_PACKAGE unnecessary for this use case.

Additional context

SDK version: 3.5.8 dpm version: 1.0.22