devtools_plugin: mark non-relative import as external
devtools_plugin: mark non-relative import as external

This is to prevent error like
https://ci.chromium.org/ui/p/devtools-frontend/builders/try/devtools_frontend_linux_blink_light_rel_fastbuild/95/overview

Bug: 1278663
Cq-Include-Trybots: luci.devtools-frontend.try:devtools_frontend_linux_blink_light_rel_fastbuild,devtools_frontend_linux_dbg_fastbuild
Change-Id: I51ee2b52b88b9b52edf47edfd94c4a771540fd89
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3430082
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Tim Van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Commit-Queue: Tim Van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

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