Search & replace package hintpaths with the solutionDir variable.

gistfile1.ps1

dir -recurse -filter *.csproj | ReplaceText-InFile -searchPattern "<HintPath>(.*)\\packages" -replaceText "<HintPath>`$(SolutionDir)\packages"

ReplaceText-Infile.ps1

    Param(
    [parameter(Mandatory=$true,ValueFromPipeline=$true)]
    [System.IO.FileInfo] $inputFile,
    [Parameter(Mandatory=$true)]
    [String] $searchPattern,
    [Parameter(Mandatory=$true)]
    [String] $replaceText
    )
    Begin{}
    Process{
       $x=gc $inputFile.FullName;
       $y=$x -replace $searchPattern, $replaceText;
       #only replace those that there actually were changes
       if (Compare-Object $x $y)
       {
        $y | Out-File -Encoding UTF8 -FilePath $inputFile.FullName
       }
     }
     End{}