Fix issues with Windows based file URIs

Previously, specifying a Windows file URI of the form 'file:///C:/foo'
as a file source failed to strip the leading slash when attempting to
source the file. Also there was ambiguity after values were munged (a
value of the form 'C:/foo' could either be a Windows file path or a
URI whose scheme is 'C').

This commit changes the file source to be more deliberate in how it
validates source properties, including only allowing absolute paths
and 'puppet' and 'file' URIs, which are both absolute and
hierarchical. Also it uses the Puppet::Util.path_to_uri method to
handle file path to URI translation issues.

Previously, if a request was created using a Windows file URI of the
form 'file:///C:/foo', then the set_uri_key method wasn't stripping
the leading slash, and setting the request key to '/C:/foo'. This
caused problems when attempting to collect metadata for Windows
files. This commit changes the request class to use the
Puppet::Util.uri_to_path method to handle URI path to file path
translation issues.

Previously, if a file URI was created programmatically using ruby's
built-in URI class, such as occurs when specifying file source URIs,
then calling URI#to_s omits the authority component, e.g. 'file:/foo'
instead of 'file:///foo'. This commit changes the URI regex to not
require two slashes, but note that the order of operations is
important as Windows file paths will match the URI regex and can be
successfully parsed: URI.parse('c:/foo').scheme == 'c'