Defaults cause hard-to-detect problems on Linux

[Jaykul]

• The PublicFilter defaults to "Public\*.ps1"
• The SourceDirectories default to "Enum", "Classes", "Private", "Public"

The result is that if someone uses lowercase "public" and "private" folders, the have to override those parameters in order for their build to work on Linux -- and if they are normally using Windows or MacOS, everything will work fine on their laptops, and only fail on a Linux build agent (see #124). The worst part is that there is no indication of what has gone wrong, because the build is simply not seeing any source files (which is considered a valid choice).

I think the best bet is to support wildcards in both of those parameters (i.e. add support to SourceDirectories). Wildcard support allows us to use [Pp] in place of P we could set the defaults to:

$PublicFilter = "[Pp]ublic\*.ps1"
$SourceDirectories = "[Ee]nums", "[Cc]lasses", "[Pp]rivate", "[Pp]ublic"

This isn't foolproof -- our default $OutputDirectory is "..\Output" which we should at least change to "../Output" -- but would resolve the most common cases where people are having problems on Linux when they use other people's templates.

[Jaykul]

To be clear, we can't just put [Pp] in SourceDirectories currently, because putting a wildcard in the path to a folder makes Get-ChildItem not get child items, and -recurse and -filter don't work as intended. We'll have to explicitly Resolve-Path on those folders before calling Get-ChildItem in the build.

Currently you can just put this in your build.psd1 to make it work with lowercase directories (on Linux, where the filesystem is case-sensitive):

SourceDirectories = 'classes','private','public'
PublicFilter = 'public/*.ps1'

[Jaykul]

It might be a good idea to do something like Get-ChildItem -filter *.ps1 -recurse and if we find any ps1 files, write a warning if we find nothing in the SourceDirectories, so it doesn't look like everything worked, when in fact, you filtered everything out.