Prevent modify package.json when plugin installation

[xuchaoying]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

bug fix

• What is the related issue? (Use #issue_id to relate an open issue)

#125

• Does this PR introduce a breaking change?

no

• Other information:

[codecov]

Codecov Report

Merging #129 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #129   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files          44       44           
  Lines        1599     1599           
=======================================
  Hits         1474     1474           
  Misses        125      125

Impacted Files	Coverage Δ
packages/svrx-util/lib/npm/index.js	96.87% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 144ab37...e1431ab. Read the comment docs.