Improve defaults for find-tag.

This implements the following heuristics for the string that
find-dot (M-.) searches for by default:

 1. when point is in an import statement,
    use the name of the imported module.
 2. when point is inside a PTS variable name,
    use that name.
 3. otherwise, provide no default.
Improve pts-module-import-at-point.

Also recognize module import with no whitespace between the module name
and the semicolon or end-of-line.
Merge pull request #93 from Toxaris/improve-find-tags

Improve defaults for find-tag.