Resolve load order ambiguities for active plugins about libloadorder

[Ortham]

Here are the results of my testing across all relevant games:

Game	sTestFile order	sTestFile fallback order	Can interleave plugins.txt and sTestFile plugins?
Oblivion	Ascending file modification timestamp	Descending filename order	Yes, plugins.txt order doesn't matter
Skyrim	Unclear: changing timestamps, filenames, sTestFile indexes and order of sTestFile entries all had no effect on load order.
Skyrim SE	Ascending file modification timestamp	Descending filename order	No, with one plugin activated by sTestFile and another by plugins.txt, their order did not change whether one had the higher timestamp or the other had it.
Fallout 3	Ascending file modification timestamp	Descending filename order	Yes, plugins.txt order doesn't matter
Fallout New Vegas	Ascending file modification timestamp	Descending filename order	Yes, plugins.txt order doesn't matter
Fallout 4	Ascending file modification timestamp	Descending filename order	No, plugins.txt is ignored when sTestFile plugins are defined
Starfield	Ascending file modification timestamp	Ascending filename order	No, plugins.txt is ignored when sTestFile plugins are defined

I double-checked Starfield's fallback order since it's different from the rest. I also double-checked Skyrim, but I can't figure it out...

[Ortham]

In the reduce-ambiguity branch I've made changes with the following effects:

• For the timestamp-based games:
  • Plugins with equal timestamps are now sorted in descending filename order, they were previously sorted in ascending filename order.
  • is_ambiguous now always returns Ok(false).
• For Skyrim and asterisk-based games:
  • Plugins that are installed but not listed in loadorder.txt (or plugins.txt, if loadorder.txt does not exist) are sorted in ascending file modification timestamp order, with files with equal timestamps being sorted in descending filename order. If a plugin's timestamp can't be read, it is sorted earlier than those that can be read. The sorted files are appended to the end of the load order (with adjustments for the usual rules like masters loading before non-masters).
  • This change affects unlisted inactive plugins as well as implicitly active plugins, but retains consistency between the two and the behaviour is now more consistent with xEdit and Wrye Bash (which sort such plugins by file modification timestamp).
• For asterisk-based games:
  • is_ambiguous() now treats the positions of all implicitly active plugins as unambiguous, rather than just early-loading plugins.

[Ortham]

Done as of 28b3ad1. I've left the behaviour for Skyrim unchanged.