patch 9.1.0359: MS-Windows: relative import in a script sourced from a buffer doesn...

patch 9.1.0359: MS-Windows: relative import in a script sourced from a buffer doesn't work

Problem:  MS-Windows: Relative import in a script sourced from a buffer
          doesn't work (Ernie Rael)
Solution: Set a filename, so that we are not trying to use
          script-relative filename (Yegappan Lakshmanan)

When a script is sourced from a buffer, the file name is set to ":source
buffer=". In MS-Windows, the ":" is a path separator character (used
after a drive letter). This results in the code trying to use the ":"
prefix to import the script on MS-Windows. To fix this, when importing a
script from a script sourced from a buffer with nofile, don't use
a script relative path name.

fixes #14588
closes: #14603

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>