fix(install): make sure to reload explicitRequests

[wraithgar]

When we are placing our deps, if our edge dep was explicitly requested,
we want to re-place it instead of leaving whatever was there before.

This will mean doing something like npm install foo will install the
latest version of foo that your tree currently allows, instead of
doing nothing if any version already existed

References

Closes npm/cli#2243

[isaacs]

LGTM once it has a test that fails without it and passes with it.

[wraithgar]

Test output if I comment out the fix in build-ideal-tree.js

  not ok must match snapshot
    --- expected                                                                                                         
    +++ actual                                                                                                           
    @@ -12,10 +12,9 @@                                                                                                   
           },                                                                                                            
           "location": "node_modules/lodash",                                                                            
           "name": "lodash",                                                                                             
           "path": "{CWD}/test/arborist/build-ideal-tree-existing-dep-that-needs-to-be-reinstalled/node_modules/lodash", 
    -      "resolved": "https://registry.npmjs.org/lodash/-/lodash-3.10.1.tgz",                                          
    -      "version": "3.10.1",                                                                                          
    +      "version": "3.9.1",                                                                                           
         },                                                                                                              
       },                                                                                                                
       "edgesOut": Map {                                                                                                 
         "lodash" => EdgeOut {                                                                                           

[isaacs]

LGTM.