Allow providing --index for directory style apps

[philippjfr]

Fixes #3486

[codecov]

Codecov Report

Merging #3493 (529fb4f) into master (59dcb37) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3493      +/-   ##
==========================================
- Coverage   83.08%   83.06%   -0.03%     
==========================================
  Files         199      199              
  Lines       27142    27147       +5     
==========================================
- Hits        22551    22549       -2     
- Misses       4591     4598       +7     

Impacted Files	Coverage Δ
panel/command/serve.py	37.05% <0.00%> (-0.85%)	⬇️
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️

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