Improve handling of multiple nested installers

[mdanish-kh]

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  • Multiple nested installers are handled improperly #371

Brief Description

1. Allocate a new installer object for each nested installer so that changes in one nested installer object do not propagate into another
2. As a consequence of the above change, logic needs to be added for merging nested installer files into a single installer in case of NestedInstallerType: portable. This is done as:
   • If two (or more) nested portable installers have the same architecture and hash, the only difference would be of RelativeFilePath in both of them; so we merge the two installers
3. Display name of NestedInstallerFile for better clarity before a prompt asking for additional metadata or asking whether the package is a portable.
4. Refactor PortableCommandAliasIfApplicable() as we expect a single NestedInstallerFiles node for each installer inside the method body now.

I've listed some test URLs/manifests in the linked issue

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Microsoft Reviewers: Open in CodeFlow

[ryfu-msft]

Tested manually with the examples in the bug issue thread. Implementation looks good. Thanks a ton!

[ryfu-msft]

/azp run

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