How to reproduce:
1. Extract the attached "1266765 repro.zip" archive directly into your drive
2. Open the "1266765 repro" project contained in the "_________________________________________117 character name__________________________________________________________" folder
3. Open the Package Manager window from Window > Package Manager
4. Select the "2D Animation" package

Expected result: an error is thrown stating that "Boris.controller.meta" file couldn't be located because the path to it exceeds the maximum path length
Actual result: a FileNotFoundException error is thrown which doesn't state what the actual issue is

Reproducible with: 2018.4.25f1, 2019.4.7f1, 2020.1.1f1, 2020.2.0a20

Notes:
-The attached archive is made to reproduce the issue on Windows as opening it results in some paths being 261 characters long which is a maximum path length limitation on Windows
-On 2018.4 stream a DirectoryNotFoundException is thrown instead
-The current error suggests that the files are actually missing instead of pointing out the actual issue which leads to confusion
-The first few lines of the error state (full error is attached in the "error.txt" file):
FileNotFoundException: Could not find file 'D:\_________________________________________117 character name__________________________________________________________\1266765 repro\Library\PackageCache\com.unity.2d.animation@5.0.0\Samples~/AnimationSamples\1 Simple\Animation\Animators\Boris.controller.meta'.
System.IO.__Error.WinIOError (System.Int32 errorCode, System.String maybeFullPath) (at <fb001e01371b4adca20013e0ac763896>:0)
System.IO.FileInfo.get_Length () (at <fb001e01371b4adca20013e0ac763896>:0)
(wrapper remoting-invoke-with-check) System.IO.FileInfo.get_Length()