Preserve requirements.psd1 and use it for managed dependencies snapshot comparison

[AnatoliB]

Potentially fixes #647 without the need to introduce a breaking change.

• When a new snapshot is created, copy the requirements.psd1 file from the app content to the snapshot directory, so that each snapshot is explicitly associated with the app author intent.
• When the worker evaluates an existing snapshot content to decide whether this snapshot is acceptable or not:
  • Compare the requirements.psd1 files in the snapshot and in the app content before looking for specific versions.
  • If the requirements.psd1 content matches, consider this snapshot acceptable (since this snapshot is the result of applying Save-Module, regardless of the module version matching rules implemented internally by Save-Module).
  • If these files don't match (or the snapshot does not contain requirements.psd1 because this is an older snapshot, created before rolling out this fix), proceed with looking for specific module versions as usual.

As a result, for example, if requirements.psd1 contains 'PnP.PowerShell' = '1.5', invoking Save-Module will still install version 1.5.0. But, since the original requirements.psd1 is preserved with the snapshot, the next PowerShell Worker instance will consider this snapshot acceptable and will start using it right away, without creating another one. The problem is solved without introducing a breaking change.

[Francisco-Gamino]

LGTM. We should add an E2E test case, which can be added in a separate PR.

[AnatoliB]

Tested on a private site extension, looks good.