Fix issues with using postcss-import plugin and postcss-modules plugin

[jsanchez034]

If you are using the postcss-import plugin, it requires that it be the first plugin passed to PostCSS (See first note in the postcss-import README). Currently rollup-plugin-postcss puts the postcss-modules plugin first in the list of PostCSS plugins causing an issue if you use CSS Modules composes from an external file together with @import syntax from the postcss-import plugin. The issue is the data from @import gets wiped out by the inlining done by composes from postcss-modules. Example..
Input

// shared/border-radius.css
.all {
  border-radius: 3px;
}

// shared/colors.css
:root {
  --white: #fff;
}

// main.css
@import '../shared/colors.css';

.test {
  composes: all from '../shared/border-radius.css';
  background-color: var(--white);
}

Output without this change

.border-radius_all__3XQLI {
  border-radius: 3px;
}
.main_test__2schg {
  border-color: var(--white);

Output with this change

.border-radius_all__3XQLI {
  border-radius: 3px;
}

:root {
  --white: #fff;
}

.main_test__2schg {
  border-color: var(--white);
}

This change makes it so that the postcss-modules plugin is last in the list of plugins passed to PostCSS.

[codecov]

Codecov Report

Merging #62 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #62   +/-   ##
=======================================
  Coverage   95.42%   95.42%           
=======================================
  Files           7        7           
  Lines         175      175           
  Branches       55       55           
=======================================
  Hits          167      167           
  Misses          8        8

Impacted Files	Coverage Δ
src/postcss-loader.js	95.16% <100%> (ø)	⬆️

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