createPluginsJs after copyCustomAdmin

[ddhp]

Description of what you did:

I was following the guide to customize Admin Panel and if instead of copying individual files but the whole ./node_modules/strapi-admin/src to ./admin/src, it wouldn't work and complain:

ERROR  Failed to compile with 7 errors                                                                                                                             2:33:25 PM

These relative modules were not found:

* ../../../strapi-plugin-content-manager/admin/src in ./.cache/admin/src/plugins.js
* ../../../strapi-plugin-content-type-builder/admin/src in ./.cache/admin/src/plugins.js
* ../../../strapi-plugin-documentation/admin/src in ./.cache/admin/src/plugins.js
* ../../../strapi-plugin-email/admin/src in ./.cache/admin/src/plugins.js
* ../../../strapi-plugin-graphql/admin/src in ./.cache/admin/src/plugins.js
* ../../../strapi-plugin-upload/admin/src in ./.cache/admin/src/plugins.js
* ../../../strapi-plugin-users-permissions/admin/src in ./.cache/admin/src/plugins.js

It's because of the .cache/admin/src/plugins.js from createPluginsJs can be potentially overwritten if plugins.js is presented at the custom admin folder.

It can be resolved by createPluginsJs after copying the custom admin folder and I think it still make sense.

Please review! Thanks!

[codecov]

Codecov Report

Merging #6626 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6626   +/-   ##
=======================================
  Coverage   19.85%   19.86%           
=======================================
  Files         857      857           
  Lines       12076    12077    +1     
  Branches     1963     1963           
=======================================
+ Hits         2398     2399    +1     
  Misses       8093     8093           
  Partials     1585     1585           

Flag	Coverage Δ
#front	14.67% <100.00%> (+<0.01%)	⬆️
#unit	41.26% <ø> (ø)

Impacted Files	Coverage Δ
...admin/src/containers/InstalledPluginsPage/index.js	7.14% <ø> (ø)
...kages/strapi-admin/admin/src/translations/index.js	100.00% <ø> (ø)
...in-content-manager/admin/src/translations/index.js	0.00% <ø> (ø)
...ntent-type-builder/admin/src/translations/index.js	0.00% <ø> (ø)
...ugin-documentation/admin/src/translations/index.js	0.00% <ø> (ø)
...trapi-plugin-email/admin/src/translations/index.js	0.00% <ø> (ø)
...rapi-plugin-upload/admin/src/translations/index.js	0.00% <ø> (ø)
...-users-permissions/admin/src/translations/index.js	0.00% <ø> (ø)
...i-admin/admin/src/containers/LocaleToggle/index.js	100.00% <100.00%> (ø)

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[soupette]

LGTM!

Thanks for taking the time to contribute!