Support evaluation of Import paths

[ntammadge]

As a user, I would like properties that are part of the path of an Import element to be evaluated as per the evaluation order in the MSBuild docs. This would allow me to execute commands like go to definition on properties originating from MSBuild files imported via paths containing properties, resulting in more accurate definition results.

To summarize the MSBuild documentation: Property and Import element evaluation occur at the same time during MSBuild's evaluation phase, evaluating values as the file is read from top to bottom.

Acceptance criteria:

• Properties should be evaluated according to any values set on lines before the property is invoked as part of the path of an Import element
• Property updates after an Import should not affect the path evaluation of the Import
• Property updates after an Import should be reflected in any Import path evaluated after the Property has been updated
• Properties evaluated should include reserved and well-known properties
• Environment variables should be evaluated during Import path resolution
• Wildcards are supported syntax for the Project attribute as documented here. Wildcards should be evaluated appropriately and result in the selection of multiple files/directories as part of a path
• Testing

[ntammadge]

Included environment variable evaluation from #18 since it makes more sense to do all of the evaluation work in a single work item